Hi Leven,
PowerBuilder can do it with HTTPClient. Or better: You can do it using HTTPClient. PowerBuilder doesn't provide special SOAP features (except the obsolete WebService Proxy project).
To create the correct request you have to read more about SOAP requests with attachments. I have never tried it but I guess you need a request with Multipart body. Such requests can contain different parts, in your case the SOAP and the attachment.
I think you can find the information you need here: https://www.w3.org/TR/SOAP-attachments/
HTH,
René