All the iPad needs (apart from the hardware changes: camera, gyro etc.) is a file "dump", where every file is (effectively) "dumped" on a file system (like the "Finder".)
Then when an app wants to access a file, the iPad asks the user, then, if the user says "yes", the app can edit that file.
Also, if you have a video that you edited with iMovie then "rendered" out, that is "dumped" in to the system; therefore, if you have an app that can colour correct movies, that app can go in to the system and edit the rendered movie.
Then, uploading a file would be easy:
1) The user click an "Upload" button (like on Mac Rumours with attachments).
2) The "dump" system is pulled up and then the user can select the file they want to upload and because all the files, even that 3rd party apps, are stored in this system, the user can select any file that they want, that was made in any app.
3) The file, that the user selects, is uploaded.