Thunderbird 15 has now U1 Filelink support to upload a file to the cloud instead of attaching it. It works great, the question is, where are the files stored - so that I can delete them later.
I did a test run sending a small file using Filelink. Than I searched inside my U1 folder - couldn't find it. Same result when searching the whole PC using the CLI.
Update - Mozilla FAQ (thanks to Tom Brossman):
Q: How long will my file be available on the storage service provider's site?
A: The file will remain available on the storage service provider's site until you specifically delete it. Log in to the provider's website to view and delete the files in your storage space.
Have you checked on the U1 web site files interface? I think Filelink creates a special folder for them on the server which isn't synchronized locally by default (unless you enabled that option), and uploads the files there via the REST API, and publishes them so that the recipient can open them.
I didn't work on this code, and don't use Thunderbird, so a bit of a guess, but this seems like what it does.