XSD is used for SOAP WS, not RESTful WS. Are you using the SOAP message format merely for encapsulating data? Then there are indeed better approaches, not least a straight HTTP file upload (along with which you can send other parameters).
and if needed send a SOAP message with a reference to the file upload.
How will the xsd look like in this case? Can you please elaborate on various options in representing the File?
If the file was uploaded by some other means (FTP, SCP, HTTP file upload, WebDAV, ...) the SOAP would just contain the name of the file and where and when it was uploaded, so that the receiving end would know where to look for it.