可预览文件类型判断，忽略大小写

app/extend/helper.js

@@ -1228,7 +1228,7 @@ module.exports = {
      * @return {Boolean} 匹配结果
      */
     canPreview(ext) {
-        const reg = /(.png)|(.gif)|(.txt)|(.jpg)|(.jpeg)|(.pdf)/;
+        const reg = /(.png)|(.gif)|(.txt)|(.jpg)|(.jpeg)|(.pdf)/i;
         return reg.test(ext);
     },