Add mail attachment download support for 2dph integration.

This gives go-onlyoffice a cookie-backed attachment download path so 2dph can reuse the canonical OnlyOffice client instead of keeping a local mail adapter.

mails.go

@@ -7,7 +7,9 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/mail"
+	"net/http"
 	"net/url"
 	"strconv"
 	"strings"
@@ -96,6 +98,38 @@ func (c *Client) GetMailMessage(ctx context.Context, messageID string) (map[stri
 	return c.ResponseObject(ctx, "/api/2.0/mail/messages/"+url.PathEscape(id))
 }
 
+// DownloadMailAttachment fetches raw attachment bytes by mail attachment id via
+// the mail addon's download.ashx handler. This path relies on the session
+// cookie captured during authentication, so NewClient configures a cookie jar.
+func (c *Client) DownloadMailAttachment(ctx context.Context, attachmentID string) ([]byte, error) {
+	id := strings.TrimSpace(attachmentID)
+	if id == "" {
+		return nil, fmt.Errorf("DownloadMailAttachment: attachment id is required")
+	}
+	auth, err := c.authHeader()
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.baseURL()+"/addons/mail/httphandlers/download.ashx?attachid="+url.QueryEscape(id), nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", auth)
+	resp, err := c.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	raw, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode >= 400 {
+		return nil, fmt.Errorf("DownloadMailAttachment %s: %d %s", id, resp.StatusCode, truncate(string(raw), 400))
+	}
+	return raw, nil
+}
+
 // RemoveMailMessages deletes messages by id (PUT /api/2.0/mail/messages/remove).
 // The API response "response" field may be a number or object; success is HTTP 2xx.
 func (c *Client) RemoveMailMessages(ctx context.Context, ids ...int) (map[string]any, error) {