1MB attachment plus empty mail body? It should be sent out if you have 20MB attachment size limit.
I changed files as you mentioned in first post, it works fine for me with Ubuntu 12.04, MySQL backend.
Usually, mail message will be encoded to base64 strings (except mail body contains just ASCII characters, no attachment, no unicode characters), and the actually file size in Postfix queue will be larger than mail header + mail body + attachment size. For example, an email with a 20MB attachment will be ABOUT 25MB (not a actual number) after encoded. So you can try to increase value of Postfix parameter "message_size_limit" and try again.
