Add centralized saveMedia helpers for image, video, and file downloads.

Platforms can register a custom saver; viewers share one download/save path instead of FileSaver call sites.

src/app/components/message/FileHeader.tsx

@@ -2,7 +2,6 @@ import { Badge, Box, IconButton, Spinner, Text, as, toRem } from 'folds';
 import { Icon, Icons } from '../icons';
 import React, { ReactNode, useCallback } from 'react';
 import { EncryptedAttachmentInfo } from 'browser-encrypt-attachment';
-import FileSaver from 'file-saver';
 import { mimeTypeToExt } from '../../utils/mimeTypes';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
@@ -14,6 +13,7 @@ import {
   mxcUrlToHttp,
 } from '../../utils/matrix';
 import { getCurrentAccessToken } from '../../utils/auth';
+import { saveMediaBlob } from '../../utils/saveMedia';
 
 const badgeStyles = { maxWidth: toRem(100) };
 
@@ -36,9 +36,8 @@ export function FileDownloadButton({ filename, url, mimeType, encInfo }: FileDow
         ? await downloadEncryptedMedia(mediaUrl, (encBuf) => decryptFile(encBuf, mimeType, encInfo), accessToken)
         : await downloadMedia(mediaUrl, accessToken);
 
-      const fileURL = URL.createObjectURL(fileContent);
-      FileSaver.saveAs(fileURL, filename);
-      return fileURL;
+      await saveMediaBlob(fileContent, filename);
+      return true;
     }, [mx, url, useAuthentication, mimeType, encInfo, filename])
   );