phab1 doesn't have that much space, only 5.5GB left and much of that space is used by /srv/phab/images
There's many old XML dumps and zip files that users uploaded for us to import that are no longer needed, and they should be deleted to save space.
I'll look into this soon though the only viable options seem to be by using CLI and the destroy function, or by changing the "ownership" of a file via SQL and then deleting it with the GUI.