@RufusVS: the open dialog is created in the function ```create_open_file_dialog()```. It calls the function ```filetypes_create_file_filter()``` to create the filter and that function is filling the filter with all filetypes from the list ```filetypes_by_title```. This is filled on startup of Geany in the function ```filetypes_init()``` which calls ```filetypes_init_types()```. Then first built-in filetypes are set in function ```init_builtin_filetypes()```. And then filetypes are added from the application data directory and the config directory. On my Ubuntu machine these are ```/usr/local/share/geany/filedefs``` and ```/home/username/.config/geany/filedefs```.
So, if I understood it right you would have to remove some files from the directory as you did **AND** shorten the list in ```init_builtin_filetypes()```and re-build Geany. Also notice that if you remove some files in the directories it will only have an effect after re-starting Geany.
But as the others wrote the list should be scrollable and it seems to be some kind of Gtk issue.