diff --git a/examples/with-koa2.js b/examples/with-koa2.js
index 1ecef737..b8cd6b62 100644
--- a/examples/with-koa2.js
+++ b/examples/with-koa2.js
@@ -11,7 +11,16 @@ app.on('error', (err) => {
 
 app.use(async (ctx, next) => {
   if (ctx.url === '/api/upload' && ctx.method.toLowerCase() === 'post') {
-    const form = formidable({ multiples: true });
+    let i = 0;
+    const form = formidable({
+      multiples: true,
+      keepExtensions: true,
+      // must return absolute path
+      filename: (part, $self) => {
+        i += 1;
+        return `${$self.uploadDir}/sasasa${i}`;
+      },
+    });
 
     // not very elegant, but that's for now if you don't want touse `koa-better-body`
     // or other middlewares.
diff --git a/src/Formidable.js b/src/Formidable.js
index c9a0d662..5d3f7a1f 100644
--- a/src/Formidable.js
+++ b/src/Formidable.js
@@ -44,11 +44,6 @@ class IncomingForm extends EventEmitter {
     this.uploaddir = dir;
     this.uploadDir = dir;
 
-    this.options.filename =
-      typeof this.options.filename === 'function'
-        ? this.options.filename.bind(this)
-        : this._uploadPath.bind(this);
-
     // initialize with null
     [
       'error',
@@ -61,6 +56,18 @@ class IncomingForm extends EventEmitter {
       this[key] = null;
     });
 
+    const hasRename = typeof this.options.filename === 'function';
+
+    if (this.options.keepExtensions === true && hasRename) {
+      this._rename = (part) => {
+        const resultFilepath = this.options.filename.call(this, part, this);
+
+        return this._uploadPath(part, resultFilepath);
+      };
+    } else {
+      this._rename = (part) => this._uploadPath(part);
+    }
+
     this._flushing = 0;
     this._fieldsSize = 0;
     this._fileSize = 0;
@@ -290,7 +297,7 @@ class IncomingForm extends EventEmitter {
     this._flushing += 1;
 
     const file = new File({
-      path: this.options.filename(part, this),
+      path: this._rename(part),
       name: part.filename,
       type: part.mime,
       hash: this.options.hash,
@@ -433,17 +440,29 @@ class IncomingForm extends EventEmitter {
     filename = filename.replace(/&#([\d]{4});/g, (_, code) =>
       String.fromCharCode(code),
     );
+
     return filename;
   }
 
-  _uploadPath(part) {
-    const name = `${this.uploadDir}${path.sep}${toHexoId()}`;
+  _getExtension(str) {
+    const basename = path.basename(str);
+    const firstDot = basename.indexOf('.');
+    const lastDot = basename.lastIndexOf('.');
+    const extname = path.extname(basename).replace(/(\.[a-z0-9]+).*/i, '$1');
 
-    if (part && this.options.keepExtensions) {
-      let ext = path.extname(typeof part === 'string' ? part : part.filename);
-      ext = ext.replace(/(\.[a-z0-9]+).*/i, '$1');
+    if (firstDot === lastDot) {
+      return extname;
+    }
+
+    return basename.slice(firstDot, lastDot) + extname;
+  }
+
+  _uploadPath(part, fp) {
+    const name = fp || `${this.uploadDir}${path.sep}${toHexoId()}`;
 
-      return `${name}${ext}`;
+    if (part && this.options.keepExtensions) {
+      const filename = typeof part === 'string' ? part : part.filename;
+      return `${name}${this._getExtension(filename)}`;
     }
 
     return name;