#21: support binary attachment downloads via /api/v1/mail/message/:uid/attachment/:filename in mail API

services/protocols/internal/application/usecase/message_render.go

@@ -110,3 +110,36 @@ func collectParts(entity *message.Entity, out *RenderedMessage) {
 		out.HTML = string(body)
 	}
 }
+
+// ExtractAttachment walks the MIME tree and returns bytes and content-type for a named attachment.
+func ExtractAttachment(entity *message.Entity, targetName string) ([]byte, string) {
+	if multipart := entity.MultipartReader(); multipart != nil {
+		for {
+			part, err := multipart.NextPart()
+			if err != nil {
+				break
+			}
+			if data, ctype := ExtractAttachment(part, targetName); data != nil {
+				return data, ctype
+			}
+		}
+	}
+
+	mediaType, _, _ := entity.Header.ContentType()
+	disposition, params, err := entity.Header.ContentDisposition()
+	name := params["filename"]
+	if name == "" {
+		_, paramsCT, _ := entity.Header.ContentType()
+		name = paramsCT["name"]
+	}
+
+	if (err == nil && disposition == "attachment" && (targetName == "" || strings.EqualFold(name, targetName))) ||
+		(targetName != "" && strings.EqualFold(name, targetName)) {
+		body, err := io.ReadAll(entity.Body)
+		if err == nil {
+			return body, mediaType
+		}
+	}
+	return nil, ""
+}
+

services/protocols/internal/presentation/http/mail_api.go

@@ -1,12 +1,15 @@
 package httppresentation
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"net/http"
 	"strconv"
 	"strings"
 
+	"github.com/emersion/go-message"
+
 	"lambdamail/protocols/internal/application/usecase"
 	"lambdamail/protocols/internal/infrastructure/postgres"
 )
@@ -99,7 +102,14 @@ func (m *mailAPI) handleMessage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	uid, err := strconv.ParseUint(strings.TrimPrefix(r.URL.Path, "/api/v1/mail/message/"), 10, 32)
+	path := strings.TrimPrefix(r.URL.Path, "/api/v1/mail/message/")
+	parts := strings.Split(path, "/")
+	if len(parts) == 0 || parts[0] == "" {
+		writeError(w, http.StatusBadRequest, "INVALID_UID", "Message UID is required")
+		return
+	}
+
+	uid, err := strconv.ParseUint(parts[0], 10, 32)
 	if err != nil {
 		writeError(w, http.StatusBadRequest, "INVALID_UID", "Message UID must be a number")
 		return
@@ -116,12 +126,43 @@ func (m *mailAPI) handleMessage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Handle attachment subpath: /api/v1/mail/message/{uid}/attachment/{filename}
+	if len(parts) >= 3 && parts[1] == "attachment" {
+		m.handleAttachmentDownload(w, r, raw, parts[2])
+		return
+	}
+
 	// Parsed here rather than in the browser: this service already depends on
 	// a MIME library, and a second parser in TypeScript would be a second set
 	// of edge cases to get wrong.
 	writeJSON(w, http.StatusOK, usecase.RenderMessage(raw, uint32(uid)))
 }
 
+func (m *mailAPI) handleAttachmentDownload(w http.ResponseWriter, _ *http.Request, raw []byte, filename string) {
+	entity, err := message.Read(bytes.NewReader(raw))
+	if err != nil {
+		writeError(w, http.StatusNotFound, "NOT_FOUND", "Attachment not found")
+		return
+	}
+
+	data, contentType := usecase.ExtractAttachment(entity, filename)
+	if data == nil {
+		writeError(w, http.StatusNotFound, "NOT_FOUND", "Attachment not found")
+		return
+	}
+
+	if contentType == "" {
+		contentType = "application/octet-stream"
+	}
+
+	w.Header().Set("Content-Type", contentType)
+	w.Header().Set("Content-Disposition", "attachment; filename=\""+filename+"\"")
+	w.Header().Set("Content-Length", strconv.Itoa(len(data)))
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(data)
+}
+
+
 func (m *mailAPI) handleSeen(w http.ResponseWriter, r *http.Request) {
 	session, ok := m.authenticate(w, r)
 	if !ok {