Can't POST attachment through Flurl Error 411 Length Required

Question

Can someone explain what I'm doing wrong here? I'm specifying the length header. No idea why its saying "Length Required". This request works fine when I send it through Postman, but fails when I send it through Flurl in C#. Heres the API docs for the call.

Error uploading attachment: Flurl.Http.FlurlHttpException: POST https://xxx.leankit.com/kanban/api/card/SaveAttachment/365784226/733054060 failed with status code 411 (Length Required).\r\nResponse body: Bad Request \nYour browser sent a request that this server could not understand.

Upload File code

public static async Task<string> UploadFile(string externalcardid, string filePath,string filename,int boardId,long size)
{
    try
    {
        var url = "https://xxx.leankit.com/kanban/api/card/SaveAttachment/" + boardId + "/" + externalcardid.ToString();
        var responseString = await url.WithBasicAuth("xxx", "yyy")
            .WithHeader("Content-Length", size)
            .WithHeader("Content-Type", "multipart/form-data")
            .PostMultipartAsync(mp=> mp
                .AddFile(filename,filePath)
            )
      .ReceiveString();
        return (responseString);
    }
    catch (Exception e)
    {
        return ("Error uploading attachment: " + e);
    }
}