From https://stackoverflow.com/questions/16086162/handle-file-download-from-ajax-post

ajax-download.js

function ab2str(buf) {
  return String.fromCharCode.apply(null, new Uint8Array(buf));
}
function ajaxDownload(url, params, successCallback, errorCallback){
	var xhr = new XMLHttpRequest();
	xhr.open('POST', url, true);
	xhr.responseType = 'arraybuffer';
	xhr.onload = function () {
	    if (this.status === 200) {
	    	var type = xhr.getResponseHeader('Content-Type');
	    	if(type == null || type.indexOf('text/html') != -1 ){
	    		if(typeof errorCallback === 'function'){
	    			console.debug(xhr.response);
	    			errorCallback(ab2str(xhr.response));
	    		}
	    		return;
	    	}else{
	    		if(typeof successCallback === 'function'){
	    			successCallback(xhr.response);
	    		}
	    	}
	        var filename = "";
	        var disposition = xhr.getResponseHeader('Content-Disposition');
	        if (disposition && disposition.indexOf('attachment') !== -1) {
	            var filenameRegex = /filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/;
	            var matches = filenameRegex.exec(disposition);
	            if (matches != null && matches[1]) filename = matches[1].replace(/['"]/g, '');
	        }
	        filename = decodeURIComponent(filename.replace(/\+/g, " "));
	        var blob = new Blob([this.response], { type: type });
	        if (typeof window.navigator.msSaveBlob !== 'undefined') {
	            // IE workaround for "HTML7007: One or more blob URLs were revoked by closing the blob for which they were created. 
	        	// These URLs will no longer resolve as the data backing the URL has been freed."
	            window.navigator.msSaveBlob(blob, filename);
	        } else {
	            var URL = window.URL || window.webkitURL;
	            var downloadUrl = URL.createObjectURL(blob);

	            if (filename) {
	                // use HTML5 a[download] attribute to specify filename
	                var a = document.createElement("a");
	                // safari doesn't support this yet
	                if (typeof a.download === 'undefined') {
	                    window.location = downloadUrl;
	                } else {
	                    a.href = downloadUrl;
	                    a.download = filename;
	                    document.body.appendChild(a);
	                    a.click();
	                }
	            } else {
	                window.location = downloadUrl;
	            }

	            setTimeout(function () { URL.revokeObjectURL(downloadUrl); }, 100); // cleanup
	        }
	    }
	};
	xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
	xhr.send($.param(params));
}