How to construct a multipart form request and POST it using the Squeak/Smalltalk WebClient

Question

I am trying to upload an image to the Twitter-API, which requires a multipart/form-data oAuth-signed POST request. However there seems to be some kind of mistake in my code, as I'm getting 503 Errors everytime I try to call the function. Which should be a "Temporarily unavailable" error, but it is constant over different times of the day and apparently other people had the same problem with uploading media via the Twitter-API.

Following some examples I found, I am currently building the multipart header like this:

postData := '--', boundary, String crlf,
            'Content-Disposition: form-data; filename="upload.png"; name="media"', String crlf,
            'Content-Type: image/png', String crlf,
            'Content-Transfer-Encoding: base64', String crlf,
            morphAsBinaryString, String crlf,
            '--', boundary,'--', String crlf.

The actual httpPost command goes like this:

request := client httpPost: url 
        content: postData
        type: 'multipart/form-data; boundary=', boundary
        do:[:req | WebUtils oAuthSign: req url: url extra: extra using: personalOauthParams].

Is this the correct approach? Thanks in advance for any help.

UPDATE: As it turns out there was a String crlf missing in the line before the media data. Now I am getting an "Authentication Required" error. Other API-calls work without problem.