Fix react warnings and part of backstage (#4706)

webapp/components/admin_console/file_upload_setting.jsx

@@ -51,7 +51,6 @@ export default class FileUploadSetting extends Setting {
             if (error) {
                 Utils.clearFileInput(this.refs.fileInput);
             }
-            this.setState({fileSelected: false, fileName: null, serverError: error});
         });
     }
 
@@ -61,7 +60,7 @@ export default class FileUploadSetting extends Setting {
             serverError = <div className='form-group has-error'><label className='control-label'>{this.state.serverError}</label></div>;
         }
 
-        var btnClass = 'btn';
+        let btnClass = 'btn';
         if (this.state.fileSelected) {
             btnClass = 'btn btn-primary';
         }