Signature problems with REST Post Upload

I'm having problems with the invalid signature error that I cannot
track down.

Environment:
  Visual Basic / .NET CF
- Creating a POST to send via the upload api at: 
http://api.flickr.com/services/upload
- Have already created infrastructure for creating / managing a token,
and have tested that it's valid.
- Post string looks like this:
(header Content-Type: "multipart/form-data; boundary=ABCDEFG")
--ABCDEFG
Content-Disposition: form-data; name="api_sig"

2cb45f28ff0ac48bdbce19eabdf7ff79
--ABCDEFG
Content-Disposition: form-data; name="api_key"

2bcbe74a0b2a64734316b32553936bfc
--ABCDEFG
Content-Disposition: form-data; name="auth_token"

1322789-f8d8485bcff01a1d
--ABCDEFG
Content-Disposition: form-data; name="description"

test2
--ABCDEFG
Content-Disposition: form-data; name="title"

test
--ABCDEFG
Content-Disposition: form-data; name="photo"
Content-Type: image/jpeg
Content-Transfer-Encoding: base64

<image_info_goes_here>
--ABCDEFG--

The signature was generated using secret: 00000001
I'm MD5-ing the following:
00000001api_key2bcbe74a0b2a64734316b32553936bfcauth_token1322789-f8d8485bcff01a1ddescriptiontest2titletest

So:
1. Is my signature correct?
2. If so, why am I getting the 'invalid signature'?

This is driving me bananas.




 
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