[TASK] Mimetype-detection without finfo_file

Function finfo_file() is only available in PHP 5.3+
or with pecl-ext fileinfo. Fall back to using
mime_content_type() instead, if necessary.

Resolves: #46126
Releases: 4.5
Change-Id: I2d0fc91396019f546467d6f9fd5718d6646614e4
Reviewed-on: https://review.typo3.org/18814
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn