We need the interface described in the main issue:

When a user Open in/with a file with the Moodle Mobile app a new popup modal window is displayed

If the user is logged in just one site, the upload can:
The user will be prompted to confirm if he wants to upload the file to his Moodle private files area

If the user is logged in more than one site, a list of available sites will be displayed. The user must choose to which site he wants to upload the file
The upload modal popup can be closed at any time (no need to wait until the upload is finished) Once the upload finished the users will be alerted by a new popup