Using normal POST (login for example) your server component get just String like:
Using multipart/form-data POST your server component get something like:
Content-type: multipart/form-data, boundary=---AaB03x
content-disposition: form-data; name="anyField"
content-disposition: form-data; name="upfile"; filename="image.jpg"
Content-Type: image/jpeg (depending on file type)
... image data...
The server component which will decode it will be a servlet in Java. The servlet may use existing classes like http://www.servlets.com/cos/javadoc/com/oreilly/servlet/MultipartRequest.html
HTTP file upload is defined by RFC1867
[ December 23, 2003: Message edited by: Torsten Schippel ]