fix: preserve original filename and extension for single-image print

Single images were converted to a client-side PDF named after the image
(photo.pdf), so the UI and submission showed a .pdf name even though the
user picked photo.png/.jpg. PDF content also cannot be renamed back to
.png/.jpg or the backend tries to decode the PDF bytes as an image.

Stop converting single images to PDF. Pass the original image straight
through (new prepareImageFile helper) so the backend renders it onto A4
itself via ConvertImageToPDF — png/jpg/jpeg keep their exact filename,
gif/webp/bmp are normalized to JPEG first since the backend only decodes
those three. N-up stays disabled for a single page, so no PDF-only code
path is reached. Batch images still merge into images.pdf as before.

Author: WiIIiamWei

src/lib/utils.ts

@@ -90,6 +90,22 @@ const normalizeImageToJpeg = (file: File): Promise<ArrayBuffer> =>
     img.src = objectUrl;
   });
 
+// Image formats the backend renders onto A4 directly. Other image types
+// (gif/webp/bmp) must be normalized to JPEG first.
+const BACKEND_IMAGE_EXTS = new Set(["png", "jpg", "jpeg"]);
+
+// Returns a single-image File ready for preview/submit: png/jpg/jpeg pass
+// through unchanged so the original filename and extension are preserved,
+// while unsupported types are normalized to JPEG.
+export const prepareImageFile = async (file: File): Promise<File> => {
+  const ext = getFileExtension(file.name);
+  if (BACKEND_IMAGE_EXTS.has(ext)) return file;
+  const buf = await normalizeImageToJpeg(file);
+  return new File([buf], file.name.replace(/\.[^.]+$/, "") + ".jpg", {
+    type: "image/jpeg",
+  });
+};
+
 export const imagesToPdf = async (files: File[]): Promise<Blob> => {
   const { PDFDocument } = await import("pdf-lib");
   const pdf = await PDFDocument.create();

src/pages/Printers.tsx

@@ -44,6 +44,7 @@ import {
   downloadFile,
   imagesToPdf,
   createNupPdf,
+  prepareImageFile,
 } from "@/lib/utils";
 import {
   isInFeishu,
@@ -667,16 +668,17 @@ function PrinterContent() {
     const merge = async () => {
       setMerging(true);
       try {
-        const blob = await imagesToPdf(imageFiles);
         if (!cancelled) {
-          // For a single image, keep the original filename (e.g. photo.jpg →
-          // photo.pdf) so the UI and submission reflect what the user picked;
-          // fall back to images.pdf only when merging multiple images.
-          const pdfName =
-            imageFiles.length === 1
-              ? imageFiles[0].name.replace(/\.[^.]+$/, "") + ".pdf"
-              : "images.pdf";
-          setFile(new File([blob], pdfName, { type: "application/pdf" }));
+          if (imageFiles.length === 1) {
+            // Single image: pass the image through unchanged (the backend
+            // renders it onto A4), so the original filename/extension is
+            // preserved instead of becoming *.pdf. Unsupported image types
+            // are normalized to JPEG first.
+            setFile(await prepareImageFile(imageFiles[0]));
+          } else {
+            const blob = await imagesToPdf(imageFiles);
+            setFile(new File([blob], "images.pdf", { type: "application/pdf" }));
+          }
         }
       } catch (err: unknown) {
         if (!cancelled) {