[BUGFIX] Multiple file upload in ElementBrowser doesn't work

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