@roka Does this work with any other Pleroma instance? I mean is the problem in compatibility with Pleroma in general or just with your site's setup? In order to figure out a problem from # side you could:
1. Lookup errors in Menu -> "Commands in a queue"
2. Turn on verbose logging with saving logs to files, and later analyse logs of a Note with an attachment sending. More on logging in AndStatus is here: https://github.com/andstatus/andstatus/issues/225