How do I create a custom Content-Type with Iron?

I want to create the header Content-Type: application/x-protobuf in my Iron web app.

I can see from the Iron docs that it's possible to construct content-types, but there's no SubLevel that corresponds to protobuf.

How can I create this content-type value?

let mut headers = Headers::new();
headers.set(ContentType(Mime(TopLevel::Application, SubLevel::???, vec![])));

but there's no SubLevel that corresponds to protobuf.

If you read the documentation for SubLevel, you'll see its definition:

pub enum SubLevel {
    Star,
    Plain,
    // ... snip ...
    Ogg,
    Ext(String),
}

Thus, you need:

extern crate iron; // 0.6.0

use iron::{
    headers::ContentType,
    mime::{Mime, SubLevel, TopLevel},
    Headers,
};

fn main() {
    let mut headers = Headers::new();
    headers.set(ContentType(Mime(
        TopLevel::Application,
        SubLevel::Ext("x-protobuf".into()),
        vec![],
    )));
}