var xmlsttr = '...';
var data = new Blob([xmlstr], {type: 'text/xml; charset=utf-8'});
var formData = new FormData();
formData.append("optionName", data, 'fileName.xml');
$.ajax({
type: "POST",
url: "/url"),
dataType: "text",
success: function() {},
error: function() {},
contentType: false,
processData: false,
data: formData
});