Use content to determine mime type

Summary:
Determine the mime type for the
extractors based on the content, not on the file
extension. This avoids feeding files with a wrong
or the same file extension into the wrong extractor.

CCBUG: 403902

Reviewers: ngraham, bruns

Reviewed By: ngraham

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18819