diff --git a/lib/extend/context.js b/lib/extend/context.js
index e6523d5f..96117a27 100644
--- a/lib/extend/context.js
+++ b/lib/extend/context.js
@@ -298,10 +298,12 @@ module.exports = {
    * @param {String} filepath
    * @param {String} filename
    */
-  download(filepath, filename) {
+  download(filepath, filename = path.basename(filepath)) {
     assert(filepath, 'filepath can not be empty');
-    this.type = path.extname(filepath);
-    filename = filename || path.basename(filepath);
+    const contentType = this.response.get('Content-Type');
+    if (!contentType) {
+      this.type = path.extname(filename);
+    }
     const contentDisposition = this.response.get('Content-Disposition');
     if (!contentDisposition) {
       this.attachment(filename);