It's the 'optimised' equivalent of (python) .pyc file. I can't find anywhere to configure the object extensions.
It's currently hard-coded: https://github.com/geany/geany-plugins/blob/1.23/treebrowser/src/treebrowser.c#L280
It's currently hard-coded:
https://github.com/geany/geany-plugins/blob/1.23/treebrowser/src/treebrowser.c#L280