If you use mod_http_upload_external, be sure to double check your iptables or firewall for any port prerouting. Otherwise, the upload of files won't happen and nothing of note will be logged in the webserver log, prosody logs and the log of the software handling the uploads.
I'm talking about these kinds of iptables rules:
-A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 22067
It's not printed in the output of iptables -L. Make
iptables dump it's config and pipe it into a pager, like this: iptables-save | less.
Big thanks to Alacer and Holger Weiß from the ejabberd@conference.process-one.net chatroom, they gave me good pointers!