One of my teammates found a work-around from this. In order insert a file into a Loop page:
- Open the file you want to insert (Doc, spreadsheet, etc)
- Use the Share button in the top right corner
- Click "Copy Link"
- Paste that link into Loop and it will insert your file
Still a problem that you can't choose the file from within Loop, but this has worked for me so far. Hope this helps you, too.