SOAP is a protocol that uses XML formatted messages and reponses.
You can use the tool SOAPUI to create such messages.
If you want to pass a file. You can do that by using a filedocument in your mapping.
Normaly you map the message to Mendix entities and create a page you that entities. See this doc
If you receive an XML file in the message you can create an import mapping and import the XML document in a microflow.