How can I change the Content-Type of the attachment in a multipart/form-data POST?

The Content-Type of the attachemt is being sent as text/plain; charset=Cp1252; name="Test Document.txt"

I am just wanting to send the attachment with Content-Type as text/plain

The application I am calling is throwing an error when I try to get a document with the SOAP generated Content-Type.  I am having no such issues with Postman or Fiddler.