XFCE is a downstream still; the likely proper place is where file and/or libmagic are made.
The upstream is _mirrored_ to GitHub [https://github.com/file/file](https://github.com/file/file), but project's readme in there lists [https://mailman.astron.com/pipermail/file](https://mailman.astron.com/pipermail/file)/ as the official mailing list and [https://bugs.astron.com](https://bugs.astron.com)/ as the official bug tracker. That's the place to start if you want to extend the file Linux utility with Nim support. The initial basic way to detect Nim source files would be to set *.nim and *.nims as such. Then the additional magic can be added - if you open /usr/share/magic and search for x-python, you'll see that the library indeed lives up to its name with the amount of effort spent to detect Python based on various Pythonic code snippets. Something similar could be done for Nim.
