fix(whatsapp): preserve outbound document filenames (#15594)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 8e0d765d1d
Co-authored-by: TsekaLuk <79151285+TsekaLuk@users.noreply.github.com>
Co-authored-by: steipete <58493+steipete@users.noreply.github.com>
Reviewed-by: @steipete

src/web/inbound/send-api.ts

@@ -38,9 +38,10 @@ export function createWebSendApi(params: {
             ...(gifPlayback ? { gifPlayback: true } : {}),
           };
         } else {
+          const fileName = sendOptions?.fileName?.trim() || "file";
           payload = {
             document: mediaBuffer,
-            fileName: "file",
+            fileName,
             caption: text || undefined,
             mimetype: mediaType,
           };