Taylor, I have a FileMaker Go application that exchanges data between the iPad and the server with imports in each direction. I typically deliver updates via email.
I always tell the users that -- after opening the file from within mail -- they must close the file and re-open it from within FileMaker Go itself. If they fail to do that, they'll get the error you describe.
And yes, it does seem that the file's location is 'unknown' until it is opened directly. I'm not aware of a solution other than closing and re-opening the file.
Thank you for confirming that I am not the only seeing this. I feel much better. I will have to communicate this to my users. Thanks again!