diff --git a/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp b/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp
--- a/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp
+++ b/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp
@@ -40,6 +40,8 @@
 #include "plugins/messagepartrenderer.h"
 #include "plugins/textmessagepartrenderer.h"
 
+#include <mimetreeparser/util.h>
+
 #include <KPluginLoader>
 
 #include <QJsonArray>
@@ -150,7 +152,15 @@
 {
     d->setup();
 
-    const auto mtName = mp->content() ? QString::fromUtf8(mp->content()->contentType()->mimeType().toLower()) : QString();
+    auto mtName = mp->content() ? QString::fromUtf8(mp->content()->contentType()->mimeType().toLower()) : QString();
+    // unfortunately there's many emails where we can't trust the attachment mimetype
+    // so try to see if we can find something better
+    if (mtName.isEmpty() || mtName == QLatin1String("application/octet-stream")) {
+        const auto textPart = mp.dynamicCast<MimeTreeParser::TextMessagePart>();
+        if (textPart)
+            mtName = MimeTreeParser::Util::mimetype(textPart->temporaryFilePath()).name();
+    }
+
     QMimeDatabase db;
     const auto mt = db.mimeTypeForName(mtName);
     auto ancestors = mt.allAncestors();
diff --git a/mimetreeparser/src/objecttreeparser.h b/mimetreeparser/src/objecttreeparser.h
--- a/mimetreeparser/src/objecttreeparser.h
+++ b/mimetreeparser/src/objecttreeparser.h
@@ -327,7 +327,7 @@
     * top-level content.
     */
     MessagePartPtr parseObjectTreeInternal(KMime::Content *node, bool mOnlyOneMimePart);
-    MessagePartPtr processType(KMime::Content *node, MimeTreeParser::ProcessResult &processResult, const QByteArray &mimeType, bool onlyOneMimePart);
+    MessagePartPtr processType(KMime::Content *node, MimeTreeParser::ProcessResult &processResult, const QString &mimeType);
 
 private:
 
diff --git a/mimetreeparser/src/objecttreeparser.cpp b/mimetreeparser/src/objecttreeparser.cpp
--- a/mimetreeparser/src/objecttreeparser.cpp
+++ b/mimetreeparser/src/objecttreeparser.cpp
@@ -56,6 +56,7 @@
 
 // Qt includes
 #include <QByteArray>
+#include <QMimeType>
 #include <QTextCodec>
 
 using namespace MimeTreeParser;
@@ -192,9 +193,9 @@
     return mParsedPart;
 }
 
-MessagePartPtr ObjectTreeParser::processType(KMime::Content *node, ProcessResult &processResult, const QByteArray &mimeType, bool onlyOneMimePart)
+MessagePartPtr ObjectTreeParser::processType(KMime::Content *node, ProcessResult &processResult, const QString &mimeType)
 {
-    const auto formatters = mSource->bodyPartFormatterFactory()->formattersForType(QString::fromUtf8(mimeType));
+    const auto formatters = mSource->bodyPartFormatterFactory()->formattersForType(mimeType);
     Q_ASSERT(!formatters.empty());
     for (auto formatter : formatters) {
         PartNodeBodyPart part(this, &processResult, mTopLevelContent, node, mNodeHelper);
@@ -258,12 +259,18 @@
 
             ProcessResult processResult(mNodeHelper);
 
-            QByteArray mimeType("text/plain");
+            QString mimeType = QStringLiteral("text/plain");
             if (node->contentType(false) && !node->contentType()->mimeType().isEmpty()) {
-                mimeType = node->contentType()->mimeType();
+                mimeType = QString::fromUtf8(node->contentType()->mimeType());
+            }
+            // unfortunately there's many emails where we can't trust the attachment mimetype
+            // so try to see if we can find something better
+            if (mimeType == QLatin1String("application/octet-stream")) {
+                const auto file = mNodeHelper->writeNodeToTempFile(node);
+                mimeType = Util::mimetype(file).name();
             }
 
-            const auto mp = processType(node, processResult, mimeType, onlyOneMimePart);
+            const auto mp = processType(node, processResult, mimeType);
             Q_ASSERT(mp);
             parsedPart->appendSubPart(mp);
             mNodeHelper->setNodeProcessed(node, false);