Post an parsed Mail-Attachment to API

I've done my first steps with Node Red and got some experiences with it.
But now I stuck on the Problem, that the file won't be send to the API.
It seems the File-Content is missing.

But let's start from the beginning... what I have so far:

First of all, I have a flow, which parse my incomming Mails to find Emails, which are invoices and have an attachment.
(This part work like a charm).

It looks like this:

So far, so good. There you see the content of the file is an buffer with the Data.

So let's jump to the next flow, who should send this file to a REST-API. Thats what it looks like:

The Function-Node "API-Template" contain the following source-code.
Because of try & error, I tried two ways to construct the Payload:

FIRST (full written):

const { payload: {attachment} } = msg;
const body = '----AaB03x\r\n'+
            'Content-Disposition: form-data; name="type"\r\n'+
            'Content-Disposition: form-data; name="file"; filename="'+attachment.filename.replace(" ","")+'"\r\n'+
            'Content-Type: '+attachment.contentType+'\r\n'+
            'Content-Transfer-Encoding: binary\r\n'+
    const headers = {};
    headers["Content-Type"]="multipart/form-data; boundary=--AaB03x";
    headers["Authorization"]="Bearer <my-secret-token-stay-here>";

return {payload:body, headers};

The Request for the First Try looks like this:

SECOND (as Object):

const { payload: {attachment} } = msg;

msg.headers = {
    'Content-Type': 'multipart/form-data',
    'Authorization': 'Bearer <my-secret-token-stay-here>',
    'Content-Length': attachment.content.length.toString()
msg.payload = {
    'file' : {
        'value': attachment.content.toString('binary'),
        'options': {
            'filename': attachment.filename.replace(" ",""),

return msg

The Request for Second-Try looks like this:

But the API-Response for both requests is: statusCode: 500.

API-Documentation notice: If no file is provided, the file upload endpoint returns HTTP status 500 .

BTW: The above Requests are piped to http-request node which do a POST to the API-Endpoint.

So the question at all: What's wrong with the way I send the file-content. Did I miss something?