@pixelwolf If you just want files with the .ino extension to be recognized as C++, just append `*.ino` to the list in [`filetype_extensions.conf`](http://geany.org/manual/index.html#filetype-extensions) (that you can edit easily using *Tools → Configuration Files → filetype_extension.conf*).
If you want to create a [custom filetype](http://geany.org/manual/index.html#custom-filetypes), you then need to name your filetype file with the `.conf` extension, like `filetypes.INO.conf` (and still need to modify `filetype_extensions.conf` to get INO files recognized form their extension).
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