Hi. I found a little bug in mod_http (shipped with Prosody 0.10.2).
To reproduce the bug, just define a custom "http_external_url" somewhere in the configuration file with a value like "https://example.org/xmpp-service/" and restart Prosody afterwards.
Now take a look into the prosody.log logfile.
[…] http_upload info URL: <https://example.org/xmpp-service/upload>; Storage path: […]
[…] http_upload info URL: <https://example.org/xmpp-serviceupload>; Storage path: […]