fix: fall back to path.extname when multi-dot extension has invalid chars

src/Formidable.js

@@ -614,6 +614,12 @@ class IncomingForm extends EventEmitter {
     const firstInvalidIndex = Array.from(rawExtname).findIndex(invalidExtensionChar);
     if (firstInvalidIndex === -1) {
       filtered = rawExtname;
+    } else if (firstDot !== lastDot) {
+      // The multi-dot extension contains invalid characters (e.g. "test(.123).pdf").
+      // Fall back to path.extname so the actual file extension is preserved.
+      const lastExtname = path.extname(basename);
+      const lastInvalidIndex = Array.from(lastExtname).findIndex(invalidExtensionChar);
+      filtered = lastInvalidIndex === -1 ? lastExtname : lastExtname.substring(0, lastInvalidIndex);
     } else {
       filtered = rawExtname.substring(0, firstInvalidIndex);
     }

test/unit/formidable.test.js

@@ -105,14 +105,24 @@ function makeHeader(originalFilename) {
     expect(ext).toBe('.QxZs');
 
     basename = getBasename('test.pdf.jqlnn<img src=a onerror=alert(1)>.png');
-    expect(basename).toHaveLength(35);
+    expect(basename).toHaveLength(29);
+    ext = path.extname(basename);
+    expect(ext).toBe('.png');
+
+    // files where the first-dot segment contains invalid chars should fall
+    // back to path.extname so the real extension is preserved (issue #980)
+    basename = getBasename({ originalFilename: 'test(.123).pdf' });
+    ext = path.extname(basename);
+    expect(ext).toBe('.pdf');
+
+    basename = getBasename({ originalFilename: 'EERS 1.1-CUR.pdf' });
     ext = path.extname(basename);
-    expect(ext).toBe('.jqlnn');
+    expect(ext).toBe('.pdf');
 
     basename = getBasename('test.<a.png');
-    expect(basename).toHaveLength(25);
+    expect(basename).toHaveLength(29);
     ext = path.extname(basename);
-    expect(ext).toBe('');
+    expect(ext).toBe('.png');
   });
 
   test(`${name}#_Array parameters support`, () => {