Why not just use the table::field in the attachment, instead of requiring a path?
Otherwise, you would have to export the container to the temporary path first, then attach it, so you're saving a step by just attaching it directly as a field reference.
FileMaker Go does not store container data in the filemaker go folder structure that we can see (temporary or documents path). If it's on the device, it's embedded where you can't access it by path, otherwise it's streaming down from a server folder store. The only way to place the content in a folder that's accessible is exporting it first.
I just tried that and it worked (sort of)
It sent the attachment from the PC, but in File Maker I had to use the insert file, insert photo didn't work
Insert File from what I can see is that if on the file you click the open in and choose file maker It will put in the file so that file maker can read it, is there a workaround to that to make it easier.