Simplify usage of QMimeDatabase.
Summary:
suffixForFileName is case insensitive, no need to lowercase the path.
Also no need to call toDisplayString, only the path matters.
Test Plan: make && ctest
Reviewers: chinmoyr, ngraham, apol
Reviewed By: ngraham, apol
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D26588