You can override the default extensions that Geany uses
for each filetype using the filetype_extensions.conf file.
To override the system-wide configuration file,
copy it from $prefix/share/geany to your
configuration directory, usually ~/.geany/.
$prefix is the path where
Geany is installed (commonly
/usr/local).
For example:
%cp /usr/local/share/geany/filetype_extensions.conf /home/username/.geany/
Then edit it and remove all the lines for filetype extensions that you
do not want to override. The remaining lines can be edited after the
= sign, using a semi-colon separated list of patterns
which should be matched for that filetype.
For example, to set the filetype extensions for Make, the
/home/username/.geany/filetype_extensions.conf
file should look like:
[Extensions]
Make=Makefile*;*.mk;Buildfile;