Accidentally hit "enter", and submitted this bug before filling out the title. Here it is:
This is a generalization of a flaw originally reported here:
Semantic Forms requires a workaround to post a template to a non-main namespace page. The workaround is to use the pagename parameter to manually fix the namespace. For the File namespace, it would look like this:
Maybe a "full page name" parameter would be the way to go in solving this. Or perhaps having an upload field contain the "File:" portion of the namespace behind the scenes.