Hi Phil,
You don't need a to register a file association to use the FilePicker to pick files from the documents folder (or anywhere else). The file association is needed (along with the Document Library Access capability) for the application to see files in the documents
folder without user intervention.
The basic rules are is that the app can see
- its own
Application Data
- files in specific typed (Music, Picture) libraries with the relevant capability
- files of types it explicitly handles in the Document library with that capability
- files explicitly chosen by the user via the FilePicker or FolderPicker.
--Rob