IMHO most of your stuff is DE agnostic. And we implemented that with our config framework in Munich.
BTW: what kind of input are you KDE people actually seeking in this ticket? Just KDE specific stuff?

• switch declaration KFileWidgetPrivate::findMatchingFilter from bool to QString … again

• const findMatchingFilter
• amend comments

• Merge test/kfilewidgettest_filter.cpp into autotests/kfilewidgettest.cpp
• Swap QCOMPARE parameters to match actual + expected output on failure
• Always test filter and file name

In D21249#467037, @dfaure wrote:

A unittest addition would be good, too.

Readd dropped lines from tests/CMakeLists.txt

• Return a QString from findMatchingFilter and handle setCurrentFilter in main function
• Add filter unit test

• Dropped the duplicate comment in matchFilter (not sure if it makes sense at all)
• Replace bool param with enum class to improve readability - should have done this from the start
• Drop const from enum as requested; I like useing const wherever possible setting preconditions…

Hope this compiles now. Since I can't test the rebased patch, I hope that is the last update.
The original working version is based on v5.44. See my previous comment.

Readd dropped QLatin1Char and use them in new code too.

This patch was developed on KIO v5.44 (git tag) in an Ubuntu 18.04 chroot (because that's my LibreOffice development environment) and rebased on master.
The test-program was run on Debian Buster via LD_PRELOAD=./git_kio/build/bin/libKF5KIOFileWidgets.so, which has otherwise KIO v5.54.