You are near to figuring everything out. But not quite. What you must try to do is to intercept byte by byte the upcoming stream or blocks by blocks. And then you must look for the boundaries with the parameter "filename". If that's the case then you are receiving the body of a file. You don't just go saving the file on the disk because it will contain the header used by the multipart/form-data protocol. Commercial upload controls implement the RFC 1867 protocol. There are VBScript code that parse form data. Look for them and port them to .NET.