Well, if this was a non-multi part message… the easiest way to remove the <string> tags is to use the Flat File Assembler to convert the Xml into flat file. i.e. make the schema called <String> with no name space. Or just use your own custom Xml Message and add your part as Xml to be converted later. Not real sure if that will work in your case.
If not, maybe try using Xml Document or XLang Any Message type rather than String.