在jQuery 1.5+ 的jqXHR下监听文件下传进度(xhr.upload)

在jQuery 1.5+ 的jqXHR上监听文件上传进度(xhr.upload)
XMLHttpRequest.upload

在Firefox, Google Chrome and Safari中，如果通过XMLHttpRequest上传文件，
是可以通过监听XMLHttpRequest.upload对象的progress事件来查看进度的。

function onprogress(evt) {    // display uploading progress infomation...};var xhr_provider = function() {    var xhr = jQuery.ajaxSettings.xhr();    if(onprogress && xhr.upload) {        xhr.upload.addEventListener('progress', onprogress, false);    }    return xhr;};$.ajax({    url: url,    timeout: 5*60*1000,    type : 'post',    data: bb.getBlob(),    contentType: 'multipart/form-data; boundary=' + boundary,    processData: false,    xhr: xhr_provider,    success: function() {},    error: function() {}});