Add helper function to determine mime type based on content and extension

Summary:
The QMimeDatabase::MatchDefault only falls back to content matching
if the extension is not known. This fails for e.g. Matroska files, where
the content allows to distinguish between audio and video files.

CCBUG: 403902

Reviewers: #baloo, #frameworks, astippich, ngraham, poboiko

Reviewed By: #baloo, astippich, ngraham

Subscribers: kde-frameworks-devel

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D20045