Replacing QMimeDatabase::mimeTypeForUrl() with FileInfo's implementation

FileInfo's implementation is more powerful, as it makes use of
QMimeDatabase::mimeTypeForUrl(..) but makes some educated
guesses on its own: for example, a URL starting with 'http'
gets MIME type 'text/html' instead of 'application/octet-stream'
(most URLs in the context of bibliographies refer to web pages
unless they have a known file extension like '.pdf' in which
case the MIME type is set to 'application/pdf').