refactor of stackoverflow answer for this - http://stackoverflow.com/questions/16086162/handle-file-download-from-ajax-post

fileXhr.js

  function getFile(link, selected) {
        var xhr = new XMLHttpRequest();
        xhr.open('POST', link, true);
        xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
        xhr.responseType = 'arraybuffer';
        xhr.onload = function () {
            if (this.status === 200) {
                formatDownload(this);
            }
        };
        xhr.send($.param(selected));
    }
 
    function formatDownload(xhr) {
        var filename = "";
        var disposition = xhr.getResponseHeader('Content-Disposition');
        if (disposition && disposition.indexOf('attachment') !== -1) {
            filename = getFileName(disposition);
        }
 
        var type = xhr.getResponseHeader('Content-Type');
        var blob = new Blob([xhr.response], { type: type });
 
        typeof window.navigator.msSaveBlob !== 'undefined' ? window.navigator.msSaveBlob(blob, filename) : downloadBlob(blob, filename);
    }
 
    function getFileName(disposition) {
        var filenameRegex = /filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/;
        var matches = filenameRegex.exec(disposition);
        if (matches != null && matches[1])
            return matches[1].replace(/['"]/g, '');
    }
       
    function downloadBlob(blob, filename) {
        var URL = window.URL || window.webkitURL;
        var downloadUrl = URL.createObjectURL(blob);
        typeof filename !== 'undefined' ? createDownloadLink(downloadUrl, filename) : window.location = downloadUrl;
        setTimeout(function () { URL.revokeObjectURL(downloadUrl); }, 100);
    }
 
    function createDownloadLink(downloadUrl, filename) {
        var a = document.createElement("a");
        typeof a.download === 'undefined' ? window.location = downloadUrl : clickDownloadLink(a, downloadUrl, filename);
    }
 
    function clickDownloadLink(a, downloadUrl, filename) {
        a.href = downloadUrl;
        a.download = filename;
        document.body.appendChild(a);
        a.click();
    }