YAML cannot be overriden like other file formats.
I went through large hurdles to implement docker-compose syntax highlighting. My problem was that I could not override the YAML file format. It seems that the main problem is that the build-in file "filetype_extensions.conf" needs to be edited, to comment out one row: #YAML=*.yaml;*.yml; otherwise it is not possible to override YAML filetype.
How to reproduce : You may see the steps necessary to make the docker-compose work in [https://wiki.geany.org/config/docker-compose%5D(https://wiki.geany.org/confi...), and if you change anything you will either break the syntax highlighting of the docker-compose or of any yaml-file which is not a docker-compose.
I get a nagging suspicion that what is called YAML in the built-in "filetype_extensions.conf" is something that cannot be overriden. Can it be a built-in from Scintilla, which is impossible to override?