Previously, when a download was aborted (e.g. due to a bad internet
connection), a partly downloaded file was remaining in cache, which
would then be delivered upon later requests.
This can lead e.g. to chats where images aren't loading.
To avoid this, first download files to a temporary file that is not the
final cache file, and only rename/move it on finish.
Note that if you already have broken downloads, you still need to clear
cache once to get rid of them after this commit, but it should not
occur anymore afterwards.
4ef1f5c90f