refactor(files): extract WriteFileDownload shared by attachment download

Split the generic file-download writer (ServeContent for seekable readers,
manual 304 + io.Copy otherwise) out of WriteAttachmentDownload so other blob
endpoints can reuse it. The attachment writer keeps its preview branch and
cache override and delegates the rest.

pkg/web/files/file.go

@@ -0,0 +1,57 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package files
+
+import (
+	"io"
+	"mime"
+	"net/http"
+	"strconv"
+
+	"code.vikunja.io/api/pkg/files"
+)
+
+// WriteFileDownload streams a loaded file (its .File reader must be open) to the
+// response as an attachment download: http.ServeContent for seekable local files
+// (Range + If-Modified-Since for free), a manual 304 + io.Copy otherwise. It does
+// not close the reader; the caller owns it.
+func WriteFileDownload(w http.ResponseWriter, r *http.Request, f *files.File) {
+	mimeToReturn := f.Mime
+	if mimeToReturn == "" {
+		mimeToReturn = "application/octet-stream"
+	}
+	w.Header().Set("Content-Disposition", mime.FormatMediaType("attachment", map[string]string{"filename": f.Name}))
+	w.Header().Set("Content-Type", mimeToReturn)
+	w.Header().Set("Content-Length", strconv.FormatUint(f.Size, 10))
+	w.Header().Set("Last-Modified", f.Created.UTC().Format(http.TimeFormat))
+
+	// Local files are *os.File (seekable), so ServeContent gives Range +
+	// If-Modified-Since for free; s3 (and the in-memory test storage) return a
+	// non-seekable reader, so check If-Modified-Since manually and io.Copy.
+	if seeker, ok := f.File.(io.ReadSeeker); ok {
+		http.ServeContent(w, r, f.Name, f.Created, seeker)
+		return
+	}
+
+	if ifModSince := r.Header.Get("If-Modified-Since"); ifModSince != "" {
+		if t, parseErr := http.ParseTime(ifModSince); parseErr == nil && !f.Created.UTC().After(t) {
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
+	}
+	_, _ = io.Copy(w, f.File)
+}

pkg/web/files/task_attachment.go

@@ -21,8 +21,6 @@
 package files
 
 import (
-	"io"
-	"mime"
 	"net/http"
 	"strconv"
 
@@ -62,9 +60,9 @@ func toAttachmentUploadError(err error) AttachmentUploadError {
 	return AttachmentUploadError{Message: err.Error()}
 }
 
-// WriteAttachmentDownload streams the attachment (or its preview) to the response:
-// http.ServeContent for seekable local files (Range + If-Modified-Since for free),
-// a manual 304 + io.Copy otherwise. It closes the file reader.
+// WriteAttachmentDownload streams the attachment (or its inline image preview) to
+// the response and closes the file reader. The non-preview path delegates to
+// WriteFileDownload, adding the cache override that lets browsers cache attachments.
 func WriteAttachmentDownload(w http.ResponseWriter, r *http.Request, ta *models.TaskAttachment, preview []byte) {
 	defer func() { _ = ta.File.File.Close() }()
 
@@ -75,30 +73,7 @@ func WriteAttachmentDownload(w http.ResponseWriter, r *http.Request, ta *models.
 		return
 	}
 
-	mimeToReturn := ta.File.Mime
-	if mimeToReturn == "" {
-		mimeToReturn = "application/octet-stream"
-	}
-	w.Header().Set("Content-Disposition", mime.FormatMediaType("attachment", map[string]string{"filename": ta.File.Name}))
-	w.Header().Set("Content-Type", mimeToReturn)
-	w.Header().Set("Content-Length", strconv.FormatUint(ta.File.Size, 10))
-	w.Header().Set("Last-Modified", ta.File.Created.UTC().Format(http.TimeFormat))
 	// Override the global no-store directive so browsers can cache attachments.
 	w.Header().Set("Cache-Control", "no-cache")
-
-	// Local files are *os.File (seekable), so ServeContent gives Range +
-	// If-Modified-Since for free; s3 (and the in-memory test storage) return a
-	// non-seekable reader, so check If-Modified-Since manually and io.Copy.
-	if seeker, ok := ta.File.File.(io.ReadSeeker); ok {
-		http.ServeContent(w, r, ta.File.Name, ta.File.Created, seeker)
-		return
-	}
-
-	if ifModSince := r.Header.Get("If-Modified-Since"); ifModSince != "" {
-		if t, parseErr := http.ParseTime(ifModSince); parseErr == nil && !ta.File.Created.UTC().After(t) {
-			w.WriteHeader(http.StatusNotModified)
-			return
-		}
-	}
-	_, _ = io.Copy(w, ta.File.File)
+	WriteFileDownload(w, r, ta.File)
 }