tm-perform-import is a command that expects the content to be already loaded from the server to the client. It does not activate any “read from server” action.
Since you wrote, that an xxx.html file could be loaded into TW I do suspect there is a permission problem.
IMO this attempt tm-xxx-import will be “blocked” by the same mechanism that prevents the _canonical_uri from loading a .tid file. I don’t know for sure, but I think the sharepoint server just “blocks the xxx.tid” GET command. I can not test atm, since I don’t have a sharepoint account atm.
I suppose, you do not want to open a browser “load file dialogue”. right?