[BUGFIX] EM: increased visual significance of Upload .t3x

Increase the visibility of upload form in "Manage extensions".

Change-Id: I6b3b563aac28d834023dc39d7d79e963f62eb23f
Fixes: #46979
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19667
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn