[BUGFIX] Handling file upload fields is broken

The handling of file upload fields in the new system extension
form was broken and not fully implemented. Fetching the
contents, type and size of the file is missing. Besides that,
sending these files via mail was not fully implemented.

Change-Id: I9ad7aa2df1e037f1ac8d50031300af9a030a617c
Fixes: #31085
Releases: 4.7, 4.6
Reviewed-on: http://review.typo3.org/6003
Reviewed-by: Peter Beernink
Tested-by: Peter Beernink
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert