[file-qml-plugin] Use Qt classes to get mimetype for files

At the moment file-qml-plugin uses an external console command ("file") to get mime type for a file.
This doesn't work due to apparmor policies:

[ 708.702747] type=1400 audit(1414674136.065:61): apparmor="DENIED" operation="exec" profile="com.ubuntu.docviewer_docviewer_0.1.37" name="/usr/bin/file" pid=5479 comm="ubuntu-docviewe" requested_mask="x" denied_mask="x" fsuid=32011 ouid=0

Use QMimeType[1] instead, from Qt5.

Marked as critical, since it's not possible to open a document on a device.

[1]: http://qt-project.org/doc/qt-5/qmimetype.html