[FEATURE] EXT:form - Add multiselect inspector editor

A new inspector editor, i.e. a new field type of the form editor,
has been added.

Additionally, this patch fixes the wrong mime types of file upload
form elements.

Resolves: #79531
Resolves: #79252
Releases: master
Change-Id: I8a4111b7daf04751bbb16a0a5946700767bc0461
Reviewed-on: https://review.typo3.org/51457


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>