Thanks for the PR! I have some things to consider:

1. I think it's time to wrap up the arguments in a record, perhaps something like type FileServerOptions req res c = { on404 :: ..., additionalHeaders :: ... }. Then we can have a defaultFileServerOptions and cause less breakage when adding new things.
2. Should the headers thing perhaps be a function instead? E.g. FilePath -> Stats -> Array (Tuple String String), that can inspect what file is being served and choose headers based on that.
3. If we decide to go with a function, as discussed in point 2, it could also take the default headers as an argument, and have the possibility to filter them out. Not sure if that is useful, though.

What do you think?

Oh, by the way, have a look at the Travis CI report. There's an example that fails to compile. You can compile them with make examples.

Okay that sounds reasonable. And by FilePath -> Stats -> Array do you mean the stats type from Node.FS.Stats?