SupportingDocument upload filename issue

[DJWalker42]

• Uploading a SupportingDocument "requires" that you supply a new filename for the upload; this must include the file extension else problems, which isn't obvious.
• If the filename field is left blank the filename should default to the current name of the file. It doesn't, the filename is left blank, thus resulting in a save failure.

We need logic to deal with user input for the filename; no file extension provided or an extension provided that does not match the file upload.

NOTICE: you won't see this problem from the GUI as the filename for the upload is (perhaps sensibly) set to the name of the uploaded file, including the file extension. If we exposed the option to set the filename in the GUI then we'd need more robust code in the API.