You can use a File Reader snap to read the file contents. Depending on your setup, you may to create a Binary Account for the snap to use to access the file (i.e. if reading from a FTP server or Windows share).
Then you can use a Binary to Document snap with no decode to make the contents available.
Finally, you use a Mapper snap, use the Base64.encode($content) function to encode the file contents and create the final envelope before calling the SOAP Execute.
This may not be complete for your example, but hopefully it gives you a good start.
Hope this helps!
