When you have a SOAPMessage Provider based CXF endpoint in your Camel route and the message is forwarded to an input stream based processor or producer (e.g., File), it is necessary to have a converter that converts SOAPMessage into InputStream.
It would be practical to have this conversion method included in CxfConverter.
I am attaching a test case and a possible implementation.
This implementation uses CachedOutputStream to make an InputStream from a SOAPMessage. There might be a better way to get an InputStream, but I don't know a simple one.