You have WM set to break apart large messages at Tools>accounts>select your
account>properties>advanced tab.
The default is set at 60KB.
You can try sending your message with the checkbox cleared.
Your ISP may have a maximum sending size limit in operation and reject
excessively large attachments?
The 31 Parts of the message that was sent can be made into one mail by
selecting each part and then on the Message menu choose 'Combine & Decode,
then double click the resultant attachment.