[BUGFIX] Call to undefined function mime_content_type()

With PHP 5.2, it might happen that mime_content_type is not
available. This was introduced by #46126 and is now better
sanitized by the patch again.

Change-Id: I4145461d46fc12d0569bfffc0ad4750f67aeed2c
Resolves: #47969
Related: #46126
Releases: 4.5
Reviewed-on: https://review.typo3.org/20557
Reviewed-by: Steffen Ritter
Reviewed-by: Jigal van Hemert
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn