HTTP multipart form sample request in plaintext and php. Make sure the boundary is correct. Notice that it must have the prefix "--" in the body but not in the header.

HTTP Multipart Form.txt

Content-type: multipart/form-data, boundary={$boundary}
Content-Length: {$contentLength}

--{$boundary}
content-disposition: form-data; name="field1"

{$field1}
--{$boundary}
content-disposition: form-data; name="field2"

{$field2}
--{$boundary}
content-disposition: form-data; name="userfile"; filename="{$filename}"
Content-Type: application/zip

{$binarydata}
--{$boundary}