Lower the log level for when the mimeFromContent differed from MimeFromExtension

Summary:
This makes ark less noisy in logs, since this message can be frequent and can be considered as info rather than a warning.
Warning log example :
ark.kerfuffle: Mimetype for filename extension ( "text/x-cmake" ) did not match mimetype for content ( "text/plain" ). Using content-based mimetype.

Reviewers: #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: kde-utils-devel, #ark

Tags: #ark

Differential Revision: https://phabricator.kde.org/D23599