Allow .nut as extension for file uploads

[Yexo]

.nut is the extension used for OpenTTD AI scripts. Complete AIs are usually packed in tar files, so no problem there, but when sharing more then a few lines of code (which can be inlined), uploading a file is better. Current workaround is to rename them to .txt before uploading.

[orudge]

.nut has been added.

[Yexo]

Thanks a lot.