Don't check if file is directory based on mime-type

Summary:
If a filename doesn't have a trailing slash, and we use QMimeDatabase::matchTypeForFile(MatchExtension) to guess its mimetype,
it results in application/octet-stream (generic type returned if QMimeDatabase wasn't able to guess) instead of inode/directory.
Because of that, indexer does try to index folders (not a big problem, but still...)

Since we've already use QFileInfo, use isDir check instead.

Test Plan: It builds. My home directory no longer pops up in UnindexedFileIterator

Reviewers: Baloo, Frameworks, bruns, astippich

Reviewed By: Baloo, bruns, astippich

Subscribers: bruns, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15960