Bug: submissions without a file extension are silently dropped

Description

Sometimes, students submit files without an extension. This should be possible, but AuTA wants an extension so that it can rename the file while retaining file format information.

Ideally, the file management portion of AuTA is reworked so that submissions have their own folder instead of a single file, so that submitted files can retain their original name.

Priority

High - students are getting "no grade"s on Submit because of this.

Definition of done

Students can submit files without a file extension.