Use file contents when selecting extractor plugin

Summary:
Selecting extractor plugin based on file name can result in failed metadata extraction if the file name suffix is wrong, e.g. a jpeg image named album.pdf.

This is part of the fix for https://bugs.kde.org/show_bug.cgi?id=220496

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: astippich, ngraham, Baloo

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D13314