Not sure if this works on iOS:
but that's the only native function I know that reads out a list of files in the documents folder.
The documentation does not state for or against if it works on iOS.
Insert From URL should not be used in your case I think. Insert File or Insert From Device are what you want.
the status function Get(DocumentsPath) is the solution to the problem and ... it works on iOS as well as on OSX!
In addition I noticed, when the insert file script step is executed automatically the inserted file is erased from the FMGo Document Folder, if you do so manually, using the dialog box, the file remains in the list. That's a bit peculiar (but o.k. as long as things work I guess)
Thanks for your help!