I'm attempting to forge content-type header.

I've attempted the followings:

XHR

request = new XMLHttpRequest();
request.open('GET', document.documentURI, false);
request.overrideMimeType('text/plain');
request.setRequestHeader('content-type', 'text/plain');
request.send();
//console.info(`all headers: ${request.getAllResponseHeaders()}`);
console.info(`content-type header: ${request.getResponseHeader('content-type')}`);
console.info(`content-type document: ${document.contentType}`);

Fetch

fetch(
  document.documentURI,
  {
    method: 'GET',
    headers: {
      "content-type" : "text/plain",
    },
  }
)
.then((response) => {
  console.info(response.headers.get('content-type'))
});

Both didn't work.

GM.xmlHttpRequest (incorporated into Promise)

gmXmlhttpRequest({
  method: 'GET',
  url: document.documentURI,
  headers: {
    "Content-Type": "text/plain",
    "Accept": "text/plain"
  },
  onprogress: function(request) {
    request.responseType = 'text';
  },
  onload: function(request) {
    request.overrideMimeType = 'text/plain';
    if (document.URL.startsWith('file:') ||
        request.status == 200) {
      myResolve(request);
    }
    else {
      myReject("File not Found");
    }
  },
  onerror: function(request) {
    console.log('Error Error Error')
  }
})

Failed.

This is currently not possible due to restrictions of web browser engine; however, an extension can overcome this issue.