jQuery的"uploadProgress“没有在"$.ajax”中触发

$.ajax({
  url: "upload.php",
  type: "POST",
  data: formData,
  processData: false,
  contentType: false,
  xhr: function() {
    var xhr = new XMLHttpRequest();
    xhr.upload.addEventListener("progress", function(evt) {
      if (evt.lengthComputable) {
        var percentComplete = Math.round((evt.loaded / evt.total) * 100);
        console.log(percentComplete + "% uploaded");
      }
    }, false);
    return xhr;
  },
  success: function(response) {
    console.log("Upload complete");
  },
  error: function(xhr, status, error) {
    console.log("Upload error: " + error);
  }
});