See the [manual](https://www.geany.org/manual/current/index.html#filetype-extensions)
This is an unhelpful comment; pointing to a sample tutorial would be better. The manual seems geared to help those already familiar with the system, and those already understanding the nuances between Filetype definitions, Custom filetypes, Special file.filetypes, and Filetype extensions. 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.