Ensure that file-create-failures gracefully resolve #640
Description
There are circumstances when a file upload will fail.
For instance, maybe the file is too large, maybe the permanent back-end failed, etc.
In these cases we want to be sure that (assuming the client properly calls things like close or delete), that we remove any temporary files.
This may already be happening, but I am confident we are leaking temp files right now and we should try to be better about this.
(We also might want to consider making sure our handles have a cleanup timeout so that if something stalls regarding a connection, we're cleaning it up after, say, 5 minutes of an open handle having non-activity.)