Forbid uploading unsupported mime types in attachment cells and add error message

[saveli]

Students simply change from "supported types" to "all types" in the file selection window and upload what they want to the attachment cells:

Unfortunately, even unsupported types are shown in the attachment overview snippet:

But when grading the attachment doesn't show and there will be an avoidable dispute.

Could you change it from only suggesting supported mime types to forbid unsupported mime types when uploading, showing an error message instead?