diff --git a/plugins/messageviewer/bodypartformatter/CMakeLists.txt b/plugins/messageviewer/bodypartformatter/CMakeLists.txt
--- a/plugins/messageviewer/bodypartformatter/CMakeLists.txt
+++ b/plugins/messageviewer/bodypartformatter/CMakeLists.txt
@@ -2,6 +2,6 @@
 add_subdirectory(autotests)
 add_subdirectory(ms-tnef)
 add_subdirectory(vcard)  
-add_subdirectory(xdiff)
+add_subdirectory(highlighter)
 add_subdirectory(calendar)
 add_subdirectory(gnupgwks)
diff --git a/plugins/messageviewer/bodypartformatter/autotests/CMakeLists.txt b/plugins/messageviewer/bodypartformatter/autotests/CMakeLists.txt
--- a/plugins/messageviewer/bodypartformatter/autotests/CMakeLists.txt
+++ b/plugins/messageviewer/bodypartformatter/autotests/CMakeLists.txt
@@ -41,4 +41,4 @@
                 )
 endmacro ()
 
-add_diff_bodyformatter_class_unittest(diffhighlightertest.cpp "../xdiff/diffhighlighter.cpp")
+add_diff_bodyformatter_class_unittest(diffhighlightertest.cpp "../highlighter/highlighter.cpp")
diff --git a/plugins/messageviewer/bodypartformatter/autotests/diffhighlightertest.cpp b/plugins/messageviewer/bodypartformatter/autotests/diffhighlightertest.cpp
--- a/plugins/messageviewer/bodypartformatter/autotests/diffhighlightertest.cpp
+++ b/plugins/messageviewer/bodypartformatter/autotests/diffhighlightertest.cpp
@@ -18,7 +18,11 @@
 */
 
 #include "diffhighlightertest.h"
-#include "../xdiff/diffhighlighter.h"
+#include "../highlighter/highlighter.h"
+
+#include <KSyntaxHighlighting/Definition>
+#include <KSyntaxHighlighting/Repository>
+#include <KSyntaxHighlighting/Theme>
 
 #include <QProcess>
 #include <QStandardPaths>
@@ -28,10 +32,6 @@
     : QObject(parent)
 {
     QStandardPaths::setTestModeEnabled(true);
-    // trick the highlighter into using the light color theme
-    auto pal = QGuiApplication::palette();
-    pal.setColor(QPalette::Base, Qt::white);
-    QGuiApplication::setPalette(pal);
 }
 
 QString readDiffFile(const QString &diffFile)
@@ -60,14 +60,18 @@
     const QString generatedFile = QStringLiteral(DIFF_DATA_DIR) + QLatin1Char('/') + input + QStringLiteral("-generated.diff");
     QString diff = readDiffFile(originalFile);
 
-    DiffHighlighter highLighter;
-    highLighter.highlightDiff(diff);
-    const QString html = highLighter.outputDiff();
-
     //Create generated file
     QFile f(generatedFile);
     QVERIFY(f.open(QIODevice::WriteOnly | QIODevice::Truncate));
-    f.write(html.toUtf8());
+    QTextStream s(&f);
+
+    KSyntaxHighlighting::Repository repo;
+    Highlighter highLighter(&s);
+    highLighter.setDefinition(repo.definitionForName(QStringLiteral("Diff")));
+    highLighter.setTheme(repo.defaultTheme(KSyntaxHighlighting::Repository::LightTheme));
+    highLighter.highlight(diff);
+
+    s.flush();
     f.close();
 
     // compare to reference file
diff --git a/plugins/messageviewer/bodypartformatter/highlighter/CMakeLists.txt b/plugins/messageviewer/bodypartformatter/highlighter/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/plugins/messageviewer/bodypartformatter/highlighter/CMakeLists.txt
@@ -0,0 +1,14 @@
+
+add_definitions(-DTRANSLATION_DOMAIN=\"messageviewer_text_xdiff_plugin\")
+set(messageviewer_bodypartformatter_text_highlighter_PART_SRCS texthighlighterplugin.cpp highlighter.cpp)
+
+add_library(messageviewer_bodypartformatter_text_highlighter MODULE ${messageviewer_bodypartformatter_text_highlighter_PART_SRCS})
+
+target_link_libraries(messageviewer_bodypartformatter_text_highlighter
+    Qt5::Core
+    KF5::MessageViewer
+    KF5::SyntaxHighlighting
+    Grantlee5::Templates
+)
+
+install(TARGETS messageviewer_bodypartformatter_text_highlighter DESTINATION ${KDE_INSTALL_PLUGINDIR}/messageviewer/bodypartformatter)
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/Messages.sh b/plugins/messageviewer/bodypartformatter/highlighter/Messages.sh
rename from plugins/messageviewer/bodypartformatter/xdiff/Messages.sh
rename to plugins/messageviewer/bodypartformatter/highlighter/Messages.sh
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.h b/plugins/messageviewer/bodypartformatter/highlighter/highlighter.h
rename from plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.h
rename to plugins/messageviewer/bodypartformatter/highlighter/highlighter.h
--- a/plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.h
+++ b/plugins/messageviewer/bodypartformatter/highlighter/highlighter.h
@@ -17,30 +17,26 @@
    Boston, MA 02110-1301, USA.
 */
 
-#ifndef DIFFHIGHLIGHTER_H
-#define DIFFHIGHLIGHTER_H
+#ifndef HIGHLIGHTER_H
+#define HIGHLIGHTER_H
 
 #include <KSyntaxHighlighting/AbstractHighlighter>
-#include <KSyntaxHighlighting/Repository>
-#include <KSyntaxHighlighting/Definition>
 
-class DiffHighlighter : public KSyntaxHighlighting::AbstractHighlighter
+class QTextStream;
+
+class Highlighter : public KSyntaxHighlighting::AbstractHighlighter
 {
 public:
-    DiffHighlighter();
-    ~DiffHighlighter();
-
-    void highlightDiff(const QString &str);
+    Highlighter(QTextStream *stream);
+    ~Highlighter();
 
-    QString outputDiff() const;
+    void highlight(const QString &str);
 
 protected:
     void applyFormat(int offset, int length, const KSyntaxHighlighting::Format &format) override;
 private:
-    KSyntaxHighlighting::Repository mRepo;
-    KSyntaxHighlighting::Definition mDef;
     QString mCurrentLine;
-    QString mOutputDiff;
+    QTextStream *mStream;
 };
 
 #endif // DIFFHIGHLIGHTER_H
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.cpp b/plugins/messageviewer/bodypartformatter/highlighter/highlighter.cpp
rename from plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.cpp
rename to plugins/messageviewer/bodypartformatter/highlighter/highlighter.cpp
--- a/plugins/messageviewer/bodypartformatter/xdiff/diffhighlighter.cpp
+++ b/plugins/messageviewer/bodypartformatter/highlighter/highlighter.cpp
@@ -17,32 +17,24 @@
    Boston, MA 02110-1301, USA.
 */
 
-#include "diffhighlighter.h"
+#include "highlighter.h"
+
 #include <KSyntaxHighlighting/Format>
-#include <KSyntaxHighlighting/Theme>
 #include <KSyntaxHighlighting/State>
+#include <KSyntaxHighlighting/Theme>
 
-#include <QGuiApplication>
-#include <QPalette>
+#include <QTextStream>
 
-DiffHighlighter::DiffHighlighter()
+Highlighter::Highlighter(QTextStream *stream)
+    : mStream(stream)
 {
-    mDef = mRepo.definitionForName(QStringLiteral("Diff"));
-    setDefinition(mDef);
-
-    setTheme(QGuiApplication::palette().color(QPalette::Base).lightness() < 128
-        ? mRepo.defaultTheme(KSyntaxHighlighting::Repository::DarkTheme)
-        : mRepo.defaultTheme(KSyntaxHighlighting::Repository::LightTheme));
 }
 
-DiffHighlighter::~DiffHighlighter()
-{
-}
+Highlighter::~Highlighter() = default;
 
-void DiffHighlighter::highlightDiff(const QString &str)
+void Highlighter::highlight(const QString &str)
 {
-    mOutputDiff.clear();
-    mOutputDiff = QStringLiteral("<pre>");
+    *mStream << QStringLiteral("<pre>");
 
     KSyntaxHighlighting::State state;
 
@@ -52,44 +44,44 @@
     for (; lineEnd != -1; lineStart = lineEnd + 1, lineEnd = str.indexOf(QLatin1Char('\n'), lineStart)) {
         mCurrentLine = str.mid(lineStart, lineEnd - lineStart);
         state = highlightLine(mCurrentLine, state);
-        mOutputDiff += QLatin1Char('\n');
+        *mStream << QLatin1Char('\n');
+    }
+    if (lineStart < str.size()) { // remaining content if str isn't ending with a newline
+       mCurrentLine = str.mid(lineStart);
+       state = highlightLine(mCurrentLine, state);
+       *mStream << QLatin1Char('\n');
     }
-    mOutputDiff += QLatin1String("</pre>\n");
+    *mStream << QLatin1String("</pre>\n");
 }
 
-void DiffHighlighter::applyFormat(int offset, int length, const KSyntaxHighlighting::Format &format)
+void Highlighter::applyFormat(int offset, int length, const KSyntaxHighlighting::Format &format)
 {
     if (!format.isDefaultTextStyle(theme())) {
-        mOutputDiff += QStringLiteral("<span style=\"");
+        *mStream << QStringLiteral("<span style=\"");
         if (format.hasTextColor(theme())) {
-            mOutputDiff += QStringLiteral("color:") + format.textColor(theme()).name() + QStringLiteral(";");
+            *mStream << QStringLiteral("color:") << format.textColor(theme()).name() << QStringLiteral(";");
         }
         if (format.hasBackgroundColor(theme())) {
-            mOutputDiff += QStringLiteral("background-color:") + format.backgroundColor(theme()).name() + QStringLiteral(";");
+            *mStream << QStringLiteral("background-color:") << format.backgroundColor(theme()).name() << QStringLiteral(";");
         }
         if (format.isBold(theme())) {
-            mOutputDiff += QStringLiteral("font-weight:bold;");
+            *mStream << QStringLiteral("font-weight:bold;");
         }
         if (format.isItalic(theme())) {
-            mOutputDiff += QStringLiteral("font-style:italic;");
+            *mStream << QStringLiteral("font-style:italic;");
         }
         if (format.isUnderline(theme())) {
-            mOutputDiff += QStringLiteral("text-decoration:underline;");
+            *mStream << QStringLiteral("text-decoration:underline;");
         }
         if (format.isStrikeThrough(theme())) {
-            mOutputDiff += QStringLiteral("text-decoration:line-through;");
+            *mStream << QStringLiteral("text-decoration:line-through;");
         }
-        mOutputDiff += QStringLiteral("\">");
+        *mStream << QStringLiteral("\">");
     }
 
-    mOutputDiff += mCurrentLine.mid(offset, length).toHtmlEscaped();
+    *mStream << mCurrentLine.mid(offset, length).toHtmlEscaped();
 
     if (!format.isDefaultTextStyle(theme())) {
-        mOutputDiff += QStringLiteral("</span>");
+        *mStream << QStringLiteral("</span>");
     }
 }
-
-QString DiffHighlighter::outputDiff() const
-{
-    return mOutputDiff;
-}
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.cpp b/plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.cpp
rename from plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.cpp
rename to plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.cpp
--- a/plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.cpp
+++ b/plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.cpp
@@ -29,19 +29,27 @@
     your version.
 */
 
-#include "diffhighlighter.h"
+#include "highlighter.h"
 
 #include <MessageViewer/MessagePartRendererBase>
 #include <MessageViewer/MessagePartRendererManager>
 #include <MessageViewer/MessagePartRenderPlugin>
 
 #include <MimeTreeParser/MessagePart>
 #include <MimeTreeParser/HtmlWriter>
 
+#include <KSyntaxHighlighting/Definition>
+#include <KSyntaxHighlighting/Repository>
+#include <KSyntaxHighlighting/Theme>
+
 #include <grantlee/template.h>
 
+#include <QGuiApplication>
+#include <QMimeDatabase>
+#include <QMimeType>
+#include <QPalette>
+
 namespace {
-// FIXME: The box should only be as wide as necessary.
 
 class Formatter : public MessageViewer::MessagePartRendererBase
 {
@@ -53,33 +61,43 @@
         if (!mp || mp->isHidden() || mp->text().isEmpty() || mp->asIcon() != MimeTreeParser::NoIcon)
             return false;
 
-        const bool diffMimeType = msgPart->content()->contentType()->mimeType() == "text/x-patch"
-                               || msgPart->content()->contentType()->mimeType() == "text/x-diff";
-        const bool diffFileName = msgPart->content()->contentType()->name().endsWith(QLatin1String(".diff"))
-                               || msgPart->content()->contentType()->name().endsWith(QLatin1String(".patch"));
-        if (!diffMimeType && !diffFileName)
+        QMimeDatabase db;
+        auto mt = db.mimeTypeForName(QString::fromLatin1(msgPart->content()->contentType()->mimeType().toLower()));
+        if (!mt.isValid())
+            return false;
+        if (mt.name() != QLatin1String("text/plain") && !mt.allAncestors().contains(QLatin1String("text/plain")))
+            return false;
+
+        const auto def = mRepo.definitionForFileName(mp->label());
+        if (!def.isValid())
             return false;
 
         auto c = MessageViewer::MessagePartRendererManager::self()->createContext();
         c.insert(QStringLiteral("block"), msgPart.data());
-        c.insert(QStringLiteral("content"), QVariant::fromValue<MessageViewer::GrantleeCallback>([msgPart](Grantlee::OutputStream *stream) {
-            DiffHighlighter highLighter;
-            highLighter.highlightDiff(msgPart->text());
-            *stream << highLighter.outputDiff();
+        c.insert(QStringLiteral("content"), QVariant::fromValue<MessageViewer::GrantleeCallback>([=](Grantlee::OutputStream*) {
+            Highlighter highLighter(htmlWriter->stream());
+            highLighter.setDefinition(def);
+            highLighter.setTheme(QGuiApplication::palette().color(QPalette::Base).lightness() < 128
+                ? mRepo.defaultTheme(KSyntaxHighlighting::Repository::DarkTheme)
+                : mRepo.defaultTheme(KSyntaxHighlighting::Repository::LightTheme));
+            highLighter.highlight(msgPart->text());
         }));
 
         auto t = MessageViewer::MessagePartRendererManager::self()->loadByName(QStringLiteral(":/textmessagepart.html"));
         Grantlee::OutputStream s(htmlWriter->stream());
         t->render(&s, &c);
         return true;
     }
+
+private:
+    mutable KSyntaxHighlighting::Repository mRepo;
 };
 
 class Plugin : public QObject, public MessageViewer::MessagePartRenderPlugin
 {
     Q_OBJECT
     Q_INTERFACES(MessageViewer::MessagePartRenderPlugin)
-    Q_PLUGIN_METADATA(IID "com.kde.messageviewer.bodypartformatter" FILE "text_xdiff.json")
+    Q_PLUGIN_METADATA(IID "com.kde.messageviewer.bodypartformatter" FILE "texthighlighterplugin.json")
 public:
     MessageViewer::MessagePartRendererBase* renderer(int index) override
     {
@@ -90,4 +108,4 @@
 };
 }
 
-#include "text_xdiff.moc"
+#include "texthighlighterplugin.moc"
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.json b/plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.json
rename from plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.json
rename to plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.json
--- a/plugins/messageviewer/bodypartformatter/xdiff/text_xdiff.json
+++ b/plugins/messageviewer/bodypartformatter/highlighter/texthighlighterplugin.json
@@ -1,3 +1,3 @@
 {
-    "renderer": [ { "type": "MimeTreeParser::AttachmentMessagePart" } ]
+    "renderer": [ { "type": "MimeTreeParser::AttachmentMessagePart", "mimetype": "text/plain" } ]
 }
diff --git a/plugins/messageviewer/bodypartformatter/xdiff/CMakeLists.txt b/plugins/messageviewer/bodypartformatter/xdiff/CMakeLists.txt
deleted file mode 100644
--- a/plugins/messageviewer/bodypartformatter/xdiff/CMakeLists.txt
+++ /dev/null
@@ -1,14 +0,0 @@
-
-add_definitions(-DTRANSLATION_DOMAIN=\"messageviewer_text_xdiff_plugin\")
-set(messageviewer_bodypartformatter_text_xdiff_PART_SRCS text_xdiff.cpp diffhighlighter.cpp)
-
-add_library(messageviewer_bodypartformatter_text_xdiff MODULE ${messageviewer_bodypartformatter_text_xdiff_PART_SRCS})
-
-target_link_libraries(messageviewer_bodypartformatter_text_xdiff
-    Qt5::Core
-    KF5::MessageViewer
-    KF5::SyntaxHighlighting
-    Grantlee5::Templates
-)
-
-install(TARGETS messageviewer_bodypartformatter_text_xdiff  DESTINATION ${KDE_INSTALL_PLUGINDIR}/messageviewer/bodypartformatter)