audiothumbnail and imagethumbnail: Mention supported mimetypes explicitly

Audiothumbnail mimetypes are listed in audiocreator.cpp.

Imagethumbnail mimetypes source: https://doc.qt.io/qt-6/qimagereader.html#supportedSubTypes

Has also fallbacks for image/* and audio/*

(cherry picked from commit b61847f4)

And add webp support as it supported https://doc.qt.io/qt-6/qtimageformats-index.html at least since Qt 6.6.

This will fix partly the issues reported https://discuss.kde.org/t/information-panel-does-not-show-png-avif-or-webp-images-just-generic-icon/23416/7 As soon as there is a standard thumbnailer for a format installed, the standard thumbnailer gets picked whenever we don’t cache (as the Information panel does). This is visible for PNG or webp as their standard thumbnailers are commonly installed.