Upload com a lib prototype para Ajax

var sendFile = function() {
               var url = 'AjaxUpLoadManutencaoUsuario';
	        var myAjax = new Ajax.Request( 
	                url,
	                {
		                method: 'post',
		                contentType: 'multipart/form-data'
		        }
	        );  
}

var sendFile = function() {
    document.frmEnviaArquivo.action="AjaxUpLoadManutencaoUsuario";
    document.frmEnviaArquivo.submit();
}

org.apache.commons.fileupload.FileUploadException: the request was rejected because no multipart boundary was found
	at org.apache.commons.fileupload.FileUploadBase$FileItemIteratorImpl.<init>(FileUploadBase.java:814)
	at org.apache.commons.fileupload.FileUploadBase.getItemIterator(FileUploadBase.java:323)
	at org.apache.commons.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:341)
	at org.apache.commons.fileupload.servlet.ServletFileUpload.parseRequest(ServletFileUpload.java:126)

if (ServletFileUpload.isMultipartContent(request)) {
   ServletFileUpload upload = new ServletFileUpload(factory);
   //define o tamanho do upload
   upload.setSizeMax(MAX_REQUEST);
   try {
        /** gera exception aqui */
       List lista = upload.parseRequest(request);
}