Ensure Backdrop deletes all files it puts into /tmp

Describe your issue or idea

Since we want Backdrop to run well on shared hosting, we should make sure it's deletes any file it saves in /tmp.

I'm finding that some shared hosting providers don't provide very much space for the /tmp directory, and it's often shared across all accounts placed on that server. If this directory is only cleared once a day by the server it's likely to fill up, which can cause a Backdrop site to become completely inoperable. (I was getting a server 500 error for all authenticated users)

1) What kind of files are placed in /tmp? 2) Are they all deleted when we're done with them?