Render file to HTML with Firefox extension if Content-Type matches

I'm trying to define a new filetype for the web, which is rendered to HTML via a browser extension. What I would like to do is have the server return files in my filetype with the HTTP header "Content-Type: text/mytype". The extension will inspect the response headers, and if the content-type matches text/mytype, it will intercept the response body and replace it with the rendered HTML. This gives us a filetype that interacts nicely with the web if you have the plugin installed.

As a proof of concept, I created a plugin working which intercepts the headers and renders them into the response, but it only works if the content-type is text/html. If I change the response content type to text/mytype on the server, Firefox downloads the file, seemingly without running my extension code at all.

Is there a different event I should be listening to, in order to intercept/change headers before and run extension code instead of just downloading the file?

Here's my code:

manifest.json:

{

  "description": "Altering HTTP responses",
  "manifest_version": 2,
  "name": "http-response-filter",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/http-response",
  "icons": {
    "48": "pen.svg"
  },

  "permissions": [
    "webRequest", "webRequestBlocking", "<all_urls>"
  ],

  "background": {
    "scripts": ["background.js"]
  },

  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "57.0a1"
    }
  }

}

background.js:

var contentType;

function contentListener(details) {
  //if(contentType === 'text/mytype') {
    let filter = browser.webRequest.filterResponseData(details.requestId);
    let decoder = new TextDecoder("utf-8");
    let encoder = new TextEncoder();

    filter.ondata = event => {
      let str = decoder.decode(event.data, {stream: true});

      // Later I want to replace this line with code that parses the
      // document and renders it to HTML
      str = str.replace(/Example/g, 'Content-Type: ' + contentType);

      filter.write(encoder.encode(str));
      filter.disconnect();
    }
  //}

  return {};
}

browser.webRequest.onBeforeRequest.addListener(
  contentListener,
  {urls: ["<all_urls>"], types: ["main_frame"]},
  ["blocking"]
);

function headersListener(details) {
  let headers = details.responseHeaders;
  for(var i = 0; i < headers.length; i++) {
    if(headers[i].name.toLowerCase() === 'content-type') {
      contentType = headers[i].value;

      // Try setting the content type to HTML to prevent downloading--
      // doesn't work
      headers[i].value = 'text/html; charset=utf-8';
    }
  }

browser.webRequest.onBeforeRequest.addListener(
  contentListener,
  {urls: ["<all_urls>"], types: ["main_frame"]},
  ["blocking"]
);
}

browser.webRequest.onHeadersReceived.addListener(
  headersListener, {
    urls: ["<all_urls>"],
    types: ["main_frame"]
  }, ["blocking", "responseHeaders"]
);