[BUGFIX] Enable file upload from Frontend context

Change-Id: Ie2bf887fc9ec49f39af1ffe85bf0aea69c29fe8c
Releases: master, 7.6
Resolves: #73154
Reviewed-on: https://review.typo3.org/46504


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>