Ensure inner el-upload uses name attribute (#3940)

In order to avoid confusion when posting the form using regular non-xhr requests.
Also do not clear input value after `handleChange` event.

packages/upload/src/upload.vue

@@ -59,7 +59,6 @@ export default {
 
       if (!files) return;
       this.uploadFiles(files);
-      this.$refs.input.value = null;
     },
     uploadFiles(files) {
       let postFiles = Array.prototype.slice.call(files);
@@ -126,6 +125,7 @@ export default {
     let {
       handleClick,
       drag,
+      name,
       handleChange,
       multiple,
       accept,
@@ -148,7 +148,7 @@ export default {
           ? <upload-dragger on-file={uploadFiles}>{this.$slots.default}</upload-dragger>
           : this.$slots.default
         }
-        <input class="el-upload__input" type="file" ref="input" on-change={handleChange} multiple={multiple} accept={accept}></input>
+        <input class="el-upload__input" type="file" ref="input" name={name} on-change={handleChange} multiple={multiple} accept={accept}></input>
       </div>
     );
   }