Google Apps Script: UrlFetchApp Post File

I'm trying to POST a file to a REST API via Google Apps Script. The idea is that I have a process that is creating copies of a Google Doc, and I want to be able to post those newly created Docs to a third party system.

I found in UrlFetchApp that I can send files. However, I'm having issues sending the correct header values.

My request looks like so:

var file = DriveApp.getFileById(fileId);
var body = {
  "file": file.getAs(MimeType.PDF)
};
var headers = {
  'Content-Disposition': 'attachment; filename="'+ file.getName() +'"',
  'Content-Length': file.getSize()
};

My options when I call UrlFetchApp.fetch(url, options) looks like :

({
  method:"POST", 
  headers:{
      'Content-Disposition':"attachment; filename=\"My Merge Development_row_1.pdf\"", 
      'Content-Length':90665, 
       Authorization:"Bearer TOKEN"
  }, 
  contentType:"application/x-www-form-urlencoded", 
  muteHttpExceptions:true, 
  payload:{file:Blob}
})

The API that I'm sending the files to requires the 'Content-Length' header. But, when I try to set a value for 'Content-Length' header I get an Apps Script error, "Attribute provided with invalid value: Header:Content-Length". If I don't set the Content-Length header then the API responds that the Content-Length and file size don't match.

Any ideas on how I set the Content-Length header so I can POST the file?

There is an existing ticket highlighting that the documentation is not clear on this very issue

The solution is:

Move content length value from "content-Length" name/value pair in headers to the advanced argument "contentLength"

So in your example your options should look like

({
  method:"POST", 
  headers:{
      'Content-Disposition':"attachment; filename=\"My Merge Development_row_1.pdf\"", 
       Authorization:"Bearer TOKEN"
  }, 
  contentLength: 90665,
  contentType:"application/x-www-form-urlencoded", 
  muteHttpExceptions:true, 
  payload:{file:Blob}
})

EDIT: Added a full example function to get contentLength and blob shown below:

function testFilePost() {
  var file = DriveApp.getFileById(doc_id).getAs(MimeType.PDF);
  var headers = {
    'Content-Disposition': 'attachment; filename="'+ file.getName() +'"',
  };
  var options =
      {
        "method" : "post",
        "payload": file.getBytes(),
        "headers": headers,
        "contentLength": file.getBytes().length,
      };
  var result = JSON.parse(UrlFetchApp.fetch('http://httpbin.org/post', options).getContentText());
  Logger.log(result);
}