How to send files via HTTP_POST with Excel using VBA?
This (using WinInet) is VB6 but should also work in VBA/Excel:
http://wqweto.wordpress.com/2011/07/12/vb6-using-wininet-to-post-binary-file/
I'm successfully using the one below (it uses ADODB.Stream and WinHttp.WinHttpRequest.5.1):
http://www.ericphelps.com/scripting/samples/reference/web/http_post.txt
(if website disappears, also available on Internet Archive)