refactor(mobile): address code review feedback

Extract APP_SCHEME_PREFIX constant for the custom URL scheme string,
and derive the photo library fallback filename extension from the
asset's MIME type instead of always using .jpg.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/app/_layout.tsx

@@ -23,6 +23,9 @@ import { ShareProvider, useShare } from "../src/context/ShareContext";
 // Helpers
 // ---------------------------------------------------------------------------
 
+/** The custom URL scheme registered in app.json. */
+const APP_SCHEME_PREFIX = "docuelevate://";
+
 /** Extract a display filename from a file:// or content:// URI. */
 function filenameFromUri(uri: string): string {
   try {
@@ -41,8 +44,7 @@ function filenameFromUri(uri: string): string {
  * itself is created once and can be easily unit-tested without a React context.
  *
  * On iOS the Share Sheet / "Open In" action may deliver the file path under
- * the app's custom URL scheme (e.g.
- * `docuelevate://private/var/mobile/Library/…/file.pdf`) instead of a plain
+ * the app's custom URL scheme (`docuelevate://…/file.pdf`) instead of a plain
  * `file://` URL.  When that happens we rewrite the URL to `file:///…` so the
  * upload logic can read the file normally.
  */
@@ -53,8 +55,8 @@ function makeUrlHandler(addPendingFile: (f: { uri: string; filename: string }) =
     // iOS may pass a filesystem path under the app's custom scheme.
     // Rewrite it to a file:// URL unless it looks like an in-app deep-link
     // (expo-router groups always start with "(").
-    if (url.startsWith("docuelevate://")) {
-      const path = url.slice("docuelevate://".length);
+    if (url.startsWith(APP_SCHEME_PREFIX)) {
+      const path = url.slice(APP_SCHEME_PREFIX.length);
       if (path.length > 0 && !path.startsWith("(")) {
         fileUri = "file:///" + path.replace(/^\/+/, "");
       }

mobile/src/screens/UploadScreen.tsx

@@ -180,7 +180,9 @@ export default function UploadScreen() {
 
     if (!result.canceled && result.assets.length > 0) {
       const asset = result.assets[0];
-      const filename = asset.fileName ?? `photo_${Date.now()}.jpg`;
+      // Derive extension from MIME type so the filename matches the actual format
+      const ext = asset.mimeType?.split("/")[1]?.replace("jpeg", "jpg") ?? "jpg";
+      const filename = asset.fileName ?? `photo_${Date.now()}.${ext}`;
       await uploadFile(asset.uri, filename, asset.mimeType ?? "image/jpeg");
     }
   }