Error when I try to sign a JPG file

I created a File document to which I attached a JPG file. When I try to sign it, I get an error.
A check should be done so that only certain types of files can be signed. For other types, display a message that says "This file cannot be signed. Only [extensions] files can be signed."

Authorized file types should be (to be confirmed by Alain):

• PDF
• DOC
• ODT
• Other ? (XLS? ODS?)