fix(mail): robust MIME filename decoding with fallback for attachments

- Add _decode_mime_filename() helper with Q-encoding fallback decoder
- Use helper for both new and existing email attachment saving
- Handles edge cases where email.header.decode_header fails on partially processed filenames

app/plugins/builtins/mail/services.py

@@ -47,6 +47,36 @@ logger = logging.getLogger(__name__)
 MAX_ATTACHMENT_SIZE = 25 * 1024 * 1024  # 25 MB
 
 
+def _decode_mime_filename(filename: str) -> str:
+    """Decode MIME-encoded filename, handling =?charset?Q?...?= and =?charset?B?...?= patterns."""
+    if not filename:
+        return "attachment"
+    # If no MIME encoding pattern, return as-is
+    if "=?" not in filename:
+        return filename
+    try:
+        from email.header import decode_header, make_header
+        return str(make_header(decode_header(filename)))
+    except Exception:
+        # Fallback: manually decode Q-encoding if decode_header fails
+        # This handles cases where the email parser partially processed the filename
+        try:
+            import re
+            def decode_q(match):
+                charset, encoding, encoded = match.group(1), match.group(2).upper(), match.group(3)
+                if encoding == 'B':
+                    import base64
+                    decoded = base64.b64decode(encoded).decode(charset or 'utf-8', errors='replace')
+                else:  # Q encoding
+                    decoded = encoded.replace('_', ' ')
+                    decoded = re.sub(r'=([0-9A-Fa-f]{2})', lambda m: chr(int(m.group(1), 16)), decoded)
+                    decoded = decoded.encode('latin-1').decode(charset or 'utf-8', errors='replace')
+                return decoded
+            return re.sub(r'=\?([^?]+)\?([BbQq])\?([^?]*)\?=', decode_q, filename)
+        except Exception:
+            return filename
+
+
 def _sanitize_filename(filename: str) -> str:
     """Sanitize a filename to prevent path traversal attacks."""
     # Remove any path components — keep only the basename
@@ -773,11 +803,7 @@ async def imap_sync_account(
                                 for att_data in attachments:
                                     try:
                                         raw_filename = att_data["filename"] or "attachment"
-                                        try:
+                                        decoded_filename = _decode_mime_filename(raw_filename)
-                                            from email.header import decode_header, make_header
-                                            decoded_filename = str(make_header(decode_header(raw_filename)))
-                                        except Exception:
-                                            decoded_filename = raw_filename
                                         storage_path = await _save_attachment_to_storage(
                                             existing_mail.id, decoded_filename, att_data["content"]
                                         )
@@ -831,11 +857,7 @@ async def imap_sync_account(
                         try:
                             # Decode MIME-encoded filenames (e.g. =?utf-8?q?...?=)
                             raw_filename = att_data["filename"] or "attachment"
-                            try:
+                            decoded_filename = _decode_mime_filename(raw_filename)
-                                from email.header import decode_header, make_header
-                                decoded_filename = str(make_header(decode_header(raw_filename)))
-                            except Exception:
-                                decoded_filename = raw_filename
                             storage_path = await _save_attachment_to_storage(
                                 mail.id, decoded_filename, att_data["content"]
                             )