The file input fields in the ElementBrowser have the attribute
multiple so you can select multiple files. But as the name
is no array it only uploads 1 file.

This patch changes the name attribute of the file input fields
to an array and shows the flashmessages when an error occurred
during upload.

Resolves: #55497
Releases: 6.2
Change-Id: I1315e35e2ef4cfa8175f962c42f32a5a33ffc16e
Reviewed-on: https://review.typo3.org/27182
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters