form_open_multipart([$action = ''[, $attributes = array()[, $hidden = array()]]]) 
Parameters:
$action (string) – Form action/target URI string 
$attributes (array) – HTML attributes 
$hidden (array) – An array of hidden fields’ definitions  Returns:
An HTML multipart form opening tag  Return type:
string  
This function is absolutely identical to form_open() above, except that it adds a multipart attribute, which is necessary if you would like to use the form to upload files with.