html

<input type="file" name="mp4" id="mp4" accept="video/mp4">
<button onclick="uploadvideo()"></button>

js

function uploadvideo() {
    var files = document.getElementById('mp4').files; //files是文件选择框选择的文件对象数组

    if(files.length == 0) return;

    var form = new FormData(),
        url = 'http://localhost:9527/uploadvideo', //服务器上传地址
        file = files[0];
    form.append('file', file);

    var xhr = new XMLHttpRequest();
    xhr.open("post", url, true);

//上传进度事件
    xhr.upload.addEventListener("progress", function(result) {
        if (result.lengthComputable) {
            //上传进度
            var percent = (result.loaded / result.total * 100).toFixed(2);
        }
    }, false);

    xhr.addEventListener("readystatechange", function() {
        var result = xhr;
        if (result.status != 200) { //error
            console.log('上传失败', result.status, result.statusText, result.response);
        }
        else if (result.readyState == 4) { //finished
            console.log('上传成功', result);
        }
    });
    xhr.send(form); //开始上传


}

flask by python3

@server.route('/uploadvideo', methods=['post'])
def upload_video():
    upload_path = '/Users/zhangxing/Downloads/test/'
    file = flask.request.files['file']
    if not file:
        return {"status":"fail"}
    filename = file.filename
    extension = filename.split('.')[-1]
    print(filename)
    if extension == 'mp4':
        file.save(os.path.join(upload_path, filename))
        result = {"status":200}
        return result
    else:
        return {"status":"extension error"}