1)Could your code work with "multipart/form-data" header type?
What do you mean? It uses the multipart/form-data header in the HTTP request and sends the HTTP request as a multipart document.

2)Wich is the max size of the file that can effectively be send?, despiting the servers limitations.
I am not sure probably the size of a VB string 64k. However, with anything bigger, VB should use swap or other mechanisms to handle the string. I am no VB guru so cannot say for sure.

3)How necessary is to do it with winsock.ocx?, it's difficult to do it through winsocks APIs?, does need winsock.ocx be packed along the aplication?
In theory the ocx should be included. If not, you need to ensure that the control is installed on the OS you are running the program on and hope that if it is installed the version is compatible withh the one you've used.

Winsock is included by default from windows 98+, so I would imagain you'd be quite safe.

4)Could your code be reproducible with Wininet APIs?
I should hope so, as this is exaclty what the Winsock control is a wrapper for