[TASK] Exclude thumbnails from screen readers in file list module

In the file list module, a file thumbnail does not contain
alternative text. To exclude these thumbnails from screen
readers and thus improve accessibility, an empty `alt`
tag is set. This applies to both the list and tile view.

Resolves: #102416
Releases: main, 12.4
Change-Id: Iaf0ce6736f669c0852f96c022fa55c128aecdb9e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82374


Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>