fix(mail): improve attachment filename and reply body handling

- Use RFC 2047 encoded-word for non-ASCII attachment filenames instead of
  RFC 2231 filename* segments, which Coremail Web fails to display.
- Build multipart messages with MIMEMultipart when attachments are present
  to keep address headers and attachment filenames Coremail-compatible.
- Add --attach support to reply and forward commands.
- Quote the original message body in reply drafts for proper threading.

bin/commands/mail.py

@@ -8,9 +8,13 @@ import re
 import smtplib
 from datetime import datetime
 from email import policy
-from email.header import decode_header
+from email.header import Header, decode_header
 from email.message import EmailMessage
+from email.mime.application import MIMEApplication
+from email.mime.multipart import MIMEMultipart
+from email.mime.text import MIMEText
 from email.parser import BytesParser
+from email.utils import parseaddr
 from pathlib import Path
 
 import click
@@ -88,6 +92,135 @@ def safe_filename(name: str) -> str:
     return name
 
 
+def _encode_filename(filename: str) -> str:
+    """Return a Content-Disposition filename value compatible with Coremail.
+
+    Coremail Web client does not reliably display RFC 2231 filename* segments.
+    We use RFC 2047 encoded-word for the plain filename parameter, which older
+    Chinese university webmail clients generally handle better.
+    """
+    ascii_name = filename.encode("ascii", "ignore").decode()
+    if ascii_name == filename:
+        return f'filename="{filename}"'
+    encoded = Header(filename, charset="utf-8").encode()
+    return f'filename="{encoded}"'
+
+
+def _quote_body(text: str) -> str:
+    """Prefix each line of a body with '> ' for reply quoting."""
+    return "\n".join(f"> {line}" for line in text.splitlines())
+
+
+def _build_reply_body(original, body: str) -> str:
+    """Build a reply body that quotes the original message."""
+    orig_body = ""
+    b = original.get_body(preferencelist=("plain",))
+    if b:
+        orig_body = b.get_content() or ""
+    quoted = _quote_body(orig_body) if orig_body else ""
+    return (
+        f"{body}\n\n"
+        f"--- Original message ---\n"
+        f"From: {original['From']}\n"
+        f"Date: {original['Date']}\n"
+        f"Subject: {original['Subject']}\n\n"
+        f"{quoted}"
+    )
+
+
+def _references(original) -> str:
+    """Build References header for a reply."""
+    refs = []
+    if original["References"]:
+        refs.append(str(original["References"]))
+    if original["Message-ID"]:
+        refs.append(str(original["Message-ID"]))
+    return " ".join(refs)
+
+
+def _build_address_header(raw_address: str) -> str:
+    """Encode a 'Display Name <addr>' header for Coremail compatibility.
+
+    Python's compat32 policy folds long encoded words incorrectly when the
+    address contains both non-ASCII display name and angle brackets. We encode
+    the display name manually and preserve the raw address outside the encoded
+    word.
+    """
+    if not raw_address:
+        return ""
+    display_name, addr_spec = parseaddr(raw_address)
+    if not display_name or display_name == addr_spec:
+        return addr_spec
+    encoded = base64.b64encode(display_name.encode("utf-8")).decode("ascii")
+    return f"=?utf-8?b?{encoded}?= <{addr_spec}>"
+
+
+def _build_message(
+    cfg: dict,
+    headers: dict,
+    body: str,
+    html: bool,
+    attach_paths,
+) -> bytes:
+    """Build a MIME message and return its serialized bytes.
+
+    Uses MIMEMultipart whenever attachments are present so that non-ASCII
+    display names in address headers and non-ASCII attachment filenames are
+    both serialized in a Coremail-compatible way.
+    """
+    attach_paths = [p for p in attach_paths if p]
+
+    if attach_paths:
+        msg = MIMEMultipart("mixed")
+        msg["From"] = cfg["email"]
+        for key, value in headers.items():
+            if key in ("To", "Cc") and value:
+                msg[key] = _build_address_header(value)
+            elif key == "Subject" and value:
+                msg[key] = Header(value, charset="utf-8").encode()
+            elif value:
+                msg[key] = value
+
+        if html:
+            text_part = MIMEText("Draft includes HTML version.", "plain", "utf-8")
+            html_part = MIMEText(body, "html", "utf-8")
+            msg.attach(text_part)
+            msg.attach(html_part)
+        else:
+            text_part = MIMEText(body, "plain", "utf-8")
+            msg.attach(text_part)
+
+        for path in attach_paths:
+            p = Path(path)
+            if not p.exists():
+                click.echo(f"Attachment not found: {path}", err=True)
+                continue
+            with open(p, "rb") as f:
+                data = f.read()
+            part = MIMEApplication(data)
+            part["Content-Disposition"] = f"attachment; {_encode_filename(p.name)}"
+            msg.attach(part)
+
+        return msg.as_bytes()
+
+    # No attachments: use EmailMessage with default policy.
+    msg = EmailMessage()
+    msg["From"] = cfg["email"]
+    for key, value in headers.items():
+        if key == "Subject" and value:
+            msg[key] = value
+        elif value:
+            msg[key] = value
+
+    if html:
+        msg.set_content("Draft includes HTML version.", subtype="plain")
+        msg.add_alternative(body, subtype="html")
+    else:
+        msg.set_content(body)
+
+    return msg.as_bytes()
+
+
 @click.group()
 def mail():
     """Mail client (IMAP/SMTP). No send command — use draft instead."""
@@ -269,35 +402,15 @@ def draft(to, cc, subject, body, attach, html):
     cfg = load_config()
     body = body.replace("\\n", "\n").replace("\\t", "\t")
 
-    msg = EmailMessage()
+    headers = {"To": to, "Subject": subject}
-    msg["From"] = cfg["email"]
-    msg["To"] = to
     if cc:
-        msg["Cc"] = cc
+        headers["Cc"] = cc
-    msg["Subject"] = subject
 
-    if html:
+    raw = _build_message(cfg, headers, body, html, attach)
-        msg.set_content("Draft email (HTML)", subtype="plain")
-        msg.add_alternative(body, subtype="html")
-    else:
-        msg.set_content(body)
-
-    for path in attach:
-        p = Path(path)
-        if not p.exists():
-            click.echo(f"Attachment not found: {path}", err=True)
-            continue
-        with open(p, "rb") as f:
-            msg.add_attachment(
-                f.read(),
-                maintype="application",
-                subtype="octet-stream",
-                filename=p.name,
-            )
 
     imap = get_imap(cfg)
     imap.select("INBOX")
-    imap.append("Drafts", None, None, msg.as_bytes())
+    imap.append("Drafts", None, None, raw)
     imap.close()
     imap.logout()
 
@@ -309,9 +422,10 @@ def draft(to, cc, subject, body, attach, html):
 @mail.command()
 @click.argument("uid", type=int)
 @click.option("--body", "-b", required=True, help="Reply body")
+@click.option("--attach", multiple=True, help="Attachment path(s)")
 @click.option("--html", is_flag=True, help="Body is HTML")
 @handle_errors
-def reply(uid, body, html):
+def reply(uid, body, attach, html):
     """Reply to a message by IMAP UID (draft mode — saves to Drafts)."""
     cfg = load_config()
     body = body.replace("\\n", "\n").replace("\\t", "\t")
@@ -327,20 +441,27 @@ def reply(uid, body, html):
         raise click.Abort()
     original = BytesParser(policy=policy.default).parsebytes(raw_orig)
 
-    msg = EmailMessage()
+    subject = original["Subject"] or ""
-    msg["From"] = cfg["email"]
+    if not str(subject).startswith("Re:"):
-    msg["To"] = original["Reply-To"] or original["From"]
+        subject = f"Re: {subject}"
-    msg["Subject"] = f"Re: {original['Subject']}" if not str(original['Subject'] or '').startswith("Re:") else str(original['Subject'])
-    msg["In-Reply-To"] = original["Message-ID"]
-    msg["References"] = original["References"] or original["Message-ID"]
 
+    headers = {
+        "To": original["Reply-To"] or original["From"],
+        "Subject": subject,
+        "In-Reply-To": original["Message-ID"],
+        "References": _references(original),
+    }
+
+    content = _build_reply_body(original, body)
     if html:
-        msg.set_content("Reply draft", subtype="plain")
+        # For HTML replies we still include the plain quoted body as plain text
-        msg.add_alternative(body, subtype="html")
+        # alternative and an HTML rendering as primary alternative.
+        content = _build_reply_body(original, body)
+        raw = _build_message(cfg, headers, content.replace("\n", "<br>\n"), True, attach)
     else:
-        msg.set_content(body)
+        raw = _build_message(cfg, headers, content, False, attach)
 
-    imap.append("Drafts", None, None, msg.as_bytes())
+    imap.append("Drafts", None, None, raw)
     imap.close()
     imap.logout()
 
@@ -351,9 +472,10 @@ def reply(uid, body, html):
 @click.argument("uid", type=int)
 @click.option("--to", required=True, help="Forward to")
 @click.option("--body", "-b", default="", help="Additional body text")
+@click.option("--attach", multiple=True, help="Attachment path(s)")
 @click.option("--html", is_flag=True, help="Body is HTML")
 @handle_errors
-def forward(uid, to, body, html):
+def forward(uid, to, body, attach, html):
     """Forward a message by IMAP UID (draft mode — saves to Drafts)."""
     cfg = load_config()
     body = body.replace("\\n", "\n").replace("\\t", "\t")
@@ -369,24 +491,28 @@ def forward(uid, to, body, html):
         raise click.Abort()
     original = BytesParser(policy=policy.default).parsebytes(raw_orig)
 
-    msg = EmailMessage()
-    msg["From"] = cfg["email"]
-    msg["To"] = to
-    msg["Subject"] = f"Fwd: {original['Subject']}"
-
     orig_body = ""
     b = original.get_body(preferencelist=("plain",))
     if b:
         orig_body = b.get_content()
 
-    if html:
+    subject = f"Fwd: {original['Subject']}"
-        msg.set_content("Forwarded message", subtype="plain")
+    content = (
-        content = f"{body}\n\n--- Forwarded message ---\nFrom: {original['From']}\nDate: {original['Date']}\nSubject: {original['Subject']}\n\n{orig_body}"
+        f"{body}\n\n"
-        msg.add_alternative(content.replace("\n", "<br>\n"), subtype="html")
+        f"--- Forwarded message ---\n"
-    else:
+        f"From: {original['From']}\n"
-        msg.set_content(f"{body}\n\n--- Forwarded message ---\nFrom: {original['From']}\nDate: {original['Date']}\nSubject: {original['Subject']}\n\n{orig_body}")
+        f"Date: {original['Date']}\n"
+        f"Subject: {original['Subject']}\n\n"
+        f"{orig_body}"
+    )
 
-    imap.append("Drafts", None, None, msg.as_bytes())
+    headers = {"To": to, "Subject": subject}
+    if html:
+        raw = _build_message(cfg, headers, content.replace("\n", "<br>\n"), True, attach)
+    else:
+        raw = _build_message(cfg, headers, content, False, attach)
+
+    imap.append("Drafts", None, None, raw)
     imap.close()
     imap.logout()