[BUGFIX] File must use MIME type from index record

[BUGFIX] File must use MIME type from index record

The MIME type is stored in a field "mimetype" in the drivers, while the
field is called "mime_type" in the database. As the file object deals
with both when retrieving the type, it must respect this mismatch.

Change-Id: I06882c4d77e38284a48f7f7d7527bfc1c535edf3
Resolves: #65335
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/37211
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>