You need to use below request format in your case:
<field name="Work Log Type">29000</field>
<field name="Description">Automation has started processing this change</field>
<field name="z2AF Work Log01">ERTQCERPRTP01:\\ERTQCERPRTP01\Downloads\CRQ00000034567.out</field>
the below "z2AF Work Log0x" fields are the fields needed for attachments. If you only have one attachment, you would omit "z2AF Work Log02" and "z2AF Work Log03".
<field id="z2AF Work Log01">null</field>
<field id="z2AF Work Log02">null</field>
<field id="z2AF Work Log03">null</field>
The value for such a fields would be the filename with full path, ex. c:\temp\myattachment.txt
It's Working as per your suggestion.