we are having rest api that does take attachment. but i don't see an option in parasoft to pass attachment with the rest client.
In the REST Client, select the Resource tab then select POST (or PUT) for the method. Next, select the Payload tab then select MIME Multipart in the box for the Payload Format. Select Table for the Input Mode. Under the table, click the Add button to add a MIME part. In the Add dialog, you specify a Name for the part as well as a Value. You should see File as one of the options for the value.