A screenshot says it best:
Notably, this is with a custom storage backend, but the current limit from what I can tell is 255 characters inside the storage engine. [See writeToEngine in PhabricatorFile](https://secure.phabricator.com/diffusion/P/browse/master/src/applications/files/storage/PhabricatorFile.php;dd1023e5a8f8aca512c798b9645e5dfe33edb87d$443).
The B2 API makes deleting files obnoxious and so I ended up having to store this special ID as the Handle.
I'm not sure if this is really worthy of a bug report, since it in theory can only be triggered by 3rd party code which is outside the rules, but it's a nit I found. Feel free to WONTFIX.