The recent change to add FileAttachment.sha256_checksum as a

cached_property was causing the files to be opened but never closed.

These would pile up, especially while running unit tests, eventually

causing operations to fail due to OS limits on the number of open files

within a process.
While tracking this down, I found a handful of other places throughout

our code where we were either definitely leaking file descriptors, or

had places where an exception could cause a leak. We also never cleaned

up the tests tempdir. This change fixes those up as well.

Ran unit tests.