Interest has been expressed on this forum concerning reading and writing files to the local device, not within the manifest. The following link indicates Google is working on this:
This article only mentions “reading” files. Hopefully the API will add writing permissions as well. (Note that writing text files in Chrome (Android or Windows) can be accomplished using link.download, search this forum for “link.download” for how to do this.) Read text files back into the app using a text object set to inputType=file.
If or when this API does appear, it would be nice to see AS incorporate this API.