g_return_val_if_fail (TNY_IS_MIME_PART (part), NULL);
header_content_type = modest_tny_mime_part_get_header_value (part, "Content-Type");
g_return_val_if_fail (TNY_IS_MIME_PART (part), NULL);
header_content_type = modest_tny_mime_part_get_header_value (part, "Content-Type");