If this method is called, the Content-Type is form-multipart and it is also the format
of the entity body, even if the file in parameter is null or empty.
Provide XPath feature : returns the list of Nodes in the xml in the entity body of the resource
in first argument, with the xPathExpression in second argument.
Provide XPath feature : returns the first of the Nodes in the xml in the entity body of the resource
in first argument, with the xPathExpression in second argument.