fix(download): fall back to filename when filename* decodes empty

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: zhanghanduo

plugins/tools/_download_runner.py

@@ -267,17 +267,21 @@ def _preflight(
 
 
 def _header_filename(headers: email.message.Message) -> str:
+    extended = ""
     fallback = ""
     for name, value in headers.get_params(header="content-disposition", failobj=[]):
         if name != "filename":
             continue
         # The parser represents filename* as an RFC 2231 tuple. Prefer it
-        # over the ordinary filename, regardless of parameter order.
+        # over the ordinary filename, regardless of parameter order, but only
+        # when it decodes to something usable: an empty filename* must not
+        # discard a valid ordinary filename.
         if isinstance(value, tuple):
-            return email.utils.collapse_rfc2231_value(value).strip()
-        if not fallback:
+            if not extended:
+                extended = email.utils.collapse_rfc2231_value(value).strip()
+        elif not fallback:
             fallback = value.strip()
-    return fallback
+    return extended or fallback
 
 
 def _safe_filename(candidate: str) -> str:

tests/test_download_filenames.py

@@ -27,6 +27,9 @@
             'attachment; filename*=UTF-8\'\'actual.pdf; filename="fallback.txt"',
             "actual.pdf",
         ),
+        ('attachment; filename="paper.pdf"; filename*=UTF-8\'\'', "paper.pdf"),
+        ("attachment; filename*=UTF-8''; filename=\"paper.pdf\"", "paper.pdf"),
+        ("attachment; filename*=UTF-8''%20%20; filename=\"paper.pdf\"", "paper.pdf"),
         ("attachment", ""),
     ],
 )
@@ -50,6 +53,7 @@ def test_response_filename(disposition: str, expected: str) -> None:
         ("", 'attachment; filename="../../outside.pdf"', "outside.pdf"),
         ("", "attachment; filename*=UTF-8''..%2Foutside.pdf", "outside.pdf"),
         ("", "attachment", "url name.pdf"),
+        ("", 'attachment; filename="paper.pdf"; filename*=UTF-8\'\'', "paper.pdf"),
         ("", "", "url name.pdf"),
         ("chosen.pdf", 'attachment; filename="report;final.pdf"', "chosen.pdf"),
     ],