"oxt" and "xpi" are actually zip files and should be uploaded to the cache.
Here is a simple fix to pick them up:
https://pagure.io/fork/dsilakov/rpkg/29796aaa021934669e90fe0504cbee94540134b3
(Failed to create a pull request, the "Request Pull" button leads me to "Page not found (404)"...)
Pull requests seems to be back, so here it is: https://pagure.io/rpkg/pull-request/23
Merged.