<form action="url" enctype="multipart/form-data" id="myform" method="post">
    <input accept="image/*" id="addfile" type="file" />
</form> 

$("#addfile").on('change', function () {
  var f = $(this).get(0).files[0];
  var form = document.getElementById('myform');
  var formData = new FormData(form);
  formData.append('Filedata', f);
  var xhr = new XMLHttpRequest();
  xhr.addEventListener("load", uploadComplete, false);
  xhr.addEventListener("error", uploadFailed, false);
  xhr.open('POST', form.action);
  xhr.send(formData);
});
function uploadComplete(evt)
{
  var data=evt.target.responseText
}
function uploadFailed()
{
  alert("上传失败！请重试！"); 
}