When using the file upload field and you tell it to just be say pdfs and jpegs, you can override this when selecting multiple items from the media library.
When you first launch the media library it is greyed out however, if you highlight multiple items it allows you to select that specific grey out item.
Maybe you mean a different type of field. I don’t see a setting in the file field type that allow multiple files to be selected.
Its when you click the file input and it brings up the media upload.
If you try and select a file from the media library that you banned it is greyed out.
If you then click before and after that and multi select from the media library it allows the restricted file to be chosen.
Yes, the uploader will let you upload other types of files but I still cannot select files that do not match the type for the file field.