HTML5 File API readAsBinaryString读取的文件与磁盘上的文件大得多，不同之处在于

  var reader = new FileReader();
  reader.onload = function(evt) {
    xhr.send(evt.target.result)
  }
  reader.readAsBinaryString(f);

multipart/form-data

FileReader

xhr.send(File)

xhr.send(FormData)

multipart/form-data

function uploadFiles(url, files) {
  var formData = new FormData();

  for (var i = 0, file; file = files[i]; ++i) {
    formData.append(file.name, file);
  }

  var xhr = new XMLHttpRequest();
  xhr.open(\'POST\', url, true);
  xhr.onload = function(e) { ... };

  xhr.send(formData);  // multipart/form-data
}

document.querySelector(\'input[type=\"file\"]\').onchange = function(e) {
  uploadFiles(\'/server\', this.files);
};