Fix: decode error in email parser app (#10920)

### What problem does this PR solve?

Fix: UnicodeDecodeError: 'gb2312' codec can't decode byte 0xab in
position 560: illegal multibyte sequence.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/email.py

@@ -62,14 +62,27 @@ def chunk(
 
     #  get the email main info
     def _add_content(msg, content_type):
+        def _decode_payload(payload, charset, target_list):
+            try:
+                target_list.append(payload.decode(charset))
+            except (UnicodeDecodeError, LookupError):
+                for enc in ["utf-8", "gb2312", "gbk", "gb18030", "latin1"]:
+                    try:
+                        target_list.append(payload.decode(enc))
+                        break
+                    except UnicodeDecodeError:
+                        continue
+                else:
+                    target_list.append(payload.decode("utf-8", errors="ignore"))
+
         if content_type == "text/plain":
-            text_txt.append(
-                msg.get_payload(decode=True).decode(msg.get_content_charset())
-            )
+            payload = msg.get_payload(decode=True)
+            charset = msg.get_content_charset() or "utf-8"
+            _decode_payload(payload, charset, text_txt)
         elif content_type == "text/html":
-            html_txt.append(
-                msg.get_payload(decode=True).decode(msg.get_content_charset())
-            )
+            payload = msg.get_payload(decode=True)
+            charset = msg.get_content_charset() or "utf-8"
+            _decode_payload(payload, charset, html_txt)
         elif "multipart" in content_type:
             if msg.is_multipart():
                 for part in msg.iter_parts():