FileGator: upload handler lets a low-privileged user touch other users' temp files

FileGator is a multi-user file manager. Prior to 7.15.0, POST /upload in backend/Controllers/UploadController.php uses the request-supplied resumableFilename as an unnamespaced key in shared temporary storage. In a multi-user deployment configured with distinct per-user home directories, a low-privileged user with upload permission can set resumableTotalChunks and resumableTotalSize to zero so the assembly loop writes no data, after which the handler reads a pre-existing temporary file with that name, stores it in the attacker's home directory, and removes it. The same filename path can append data when chunks are supplied, allowing disclosure, modification, or deletion of another user's transient object, including a pending archive or the login rate-limit lockfile, while Tmpfs path sanitization limits the primitive to the temporary directory. This issue is fixed in version 7.15.0.