Bugfix: Remove FileAnnotations when multiple images are selected

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/11885#comment:3
FileAnnotation removal wasn't handled correctly when multiple images were selected

To Test:
Attach a FileAnnotation to three images. Select two images and click on the "Remove all attachments" icon. You should not see a warning, and the attachment should just be removed from these two images. Attach the file again to both images via attachment -> "Uploaded document". Now select all three images and click on "Remove all attachments" again. Now you should see a warning, that the attachment will be deleted. Proceed and check a) that the attachment was removed from all three images and b) that the file was deleted (attachment -> "Uploaded document": Available section should not contain the file anymore).