/*
 * call-seq:
 *   Curl::PostField.file(name, local_file_name) => #<Curl::PostField...>
 *   Curl::PostField.file(name, local_file_name, remote_file_name = local_file_name) => #<Curl::PostField...>
 *   Curl::PostField.file(name, remote_file_name) { |field| ... } => #<Curl::PostField...>
 * 
 * Create a new Curl::PostField for a file upload field, supplying the local filename
 * to read from, and optionally the remote filename (defaults to the local name).
 * 
 * The block form allows a block to supply the content for this field, called
 * during the perform. The block should return a ruby string with the field
 * data.
 */
static VALUE ruby_curl_postfield_new_file(int argc, VALUE *argv, VALUE klass) {