feat(upload): before 选项的返回值支持 Promise 和 Deferred Promise 对象类型 (#1919)

* feat(upload): before 选项的返回值支持 Promise 和 Deferred Promise 对象类型

* docs(upload): 更新文档

docs/upload/detail/options.md

@@ -388,6 +388,37 @@ before: function(obj){ // obj 参数同 choose
   }
   */
 }
+
+// 返回 jQuery Deferred.promise 对象或 JS 原生 Promise 对象，reject 表明阻止上传(2.9.11+)
+// Promise
+before: function(obj){
+  return new Promise(function(resolve, reject){
+    setTimeout(function(){
+      console.log('before_async_task', obj);
+      resolve();
+    }, 1000)
+  })
+}
+
+// Deferred
+before: function(obj){
+  return $.Deferred(function(deferred){
+    setTimeout(function(){
+      console.log('before_async_task', obj);
+      deferred.resolve();
+    }, 1000)
+  }).promise();
+}
+
+// Deferred2
+before: function(obj){
+  var deferred = $.Deferred();
+  setTimeout(function(){
+    console.log('before_async_task', obj);
+    deferred.resolve();
+  }, 1000)
+  return deferred.promise();
+}
 ```
 
 </td>