I'm wrestling with the same thing as I want a hosted file to use an External Data Source reference to correctly link to a local file on the iOS device. Once I manually find and select the file, it works, but not until then...
I can tell that it is due to the file structure on the two machines being different. If I find the right string to use for this--apparently a relative path reference doesn't cut it in my case, I'll try to let you know.
Perhaps another user already has this info and can share?