The default transcoding configuration of Opus lists oga as its targetFormat.
oga is one possible file name extension for the Ogg container format and thus could also mean Vorbis, Theora, or even FLAC.
Please change Opus' targetFormat from oga to opus so clients do not have to guess.

I've changed this on my own server manually via EnableTranscodingConfig, but I think this change should be the default.