Resolution:
Use the InAttachments as the in parameter in the send activity and pass the path of the file to be attached relative to the commondir as follows:

&ltActivity Name="SendEmail">
        &ltAction&gtSend</Action>
        &ltDescription&gtSend an email</Description>
        &ltExecution&gtASYNCHR</Execution>
        &ltParameter direction="in" eval="constant" type="string" name="Protocol"&gtSMTP</Parameter>
        &ltParameter direction="in" eval="constant" type="string" name="Address"&gtxyz@tibco.com</Parameter>
        &ltParameter direction="in" eval="constant" name="Sender" type="string"&gtabc@tibco.com</Parameter>
        &ltParameter direction="in" eval="constant" name="Form" type="string"&gtstandard/forms/fm26ca.xml</Parameter>
        &ltParameter direction="in" eval="variable" name="InDocument" type="document"&gtworkDoc</Parameter>
        &ltParameter direction="in" eval="constant" name="Presentation" type="string"&gtcom.martquest.eml.html.EmailProductEditTimeOut</Parameter>
        &ltParameter direction="in" eval="constant" name="InAttachments" type="string">/Temp/customUtil</Parameter>
</Activity>