let formData = new FormData(), file = $("#File")[0].files[0];

if (undefined !== file) {

    formData.append("File", $("#File")[0].files[0]);

}

 $.ajax({

    url: "",

    type: 'post',

    data: formData,

    processData: false,

    // 告诉jQuery不要去设置Content-Type请求头

    contentType: false,

    success: function (json) {

    }

});