I'm not sure which file to modify to add .mjs as an extension for javascript.
The system [filetype_extensions.conf](https://github.com/geany/geany/blob/master/data/filetype_extensions.conf) that you edited locally.
This is an unhelpful comment; pointing to a sample tutorial would be better.
If someone wrote one they could put it on the wiki and post a link here.
I am currently trying to add .awk extension to look like .pl, and having problems. I've created entry [Extensions] + Awk=*.awk; in filetype_extensions.conf, created in ~/.config/geany/filedefs. Also a filetypes.awk there, copied from /usr/shared/geany/filedefs/filetypes.perl and changing perl & pl to awk but ... not working.
If you want awk to be treated like perl, can't you just add `*.awk` to the perl line?