[BUGFIX] Adding new image to content element breaks layout

Since #55545 the frameset layout behaves strange in
Chrome and Chromium when you add a new image to
a content element. This is caused by some css set
on the hidden input[file] element.

This patch changes the css to prevent this behaviour.

Resolves: #56018
Releases: 6.2
Change-Id: I8a9b6b7d402ec712cb9b1761b059d2eeebd0ad3e
Reviewed-on: https://review.typo3.org/27655
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind