diff --git a/src/domain/session/room/timeline/tiles/FileTile.js b/src/domain/session/room/timeline/tiles/FileTile.js
index c761e68e..1007d28c 100644
--- a/src/domain/session/room/timeline/tiles/FileTile.js
+++ b/src/domain/session/room/timeline/tiles/FileTile.js
@@ -16,7 +16,7 @@ limitations under the License.
 */
 
 import {BaseMessageTile} from "./BaseMessageTile.js";
-import {formatSize} from "../../../../../utils/formatSize.js";
+import {formatSize} from "../../../../../utils/formatSize";
 import {SendStatus} from "../../../../../matrix/room/sending/PendingEvent.js";
 
 export class FileTile extends BaseMessageTile {
diff --git a/src/utils/formatSize.js b/src/utils/formatSize.ts
similarity index 92%
rename from src/utils/formatSize.js
rename to src/utils/formatSize.ts
index d001dacd..14ec6387 100644
--- a/src/utils/formatSize.js
+++ b/src/utils/formatSize.ts
@@ -14,7 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-export function formatSize(size, decimals = 2) {
+
+export function formatSize(size: number, decimals: number = 2): string | undefined {
     if (Number.isSafeInteger(size)) {
         const base = Math.min(3, Math.floor(Math.log(size) / Math.log(1024)));
         const formattedSize = Math.round(size / Math.pow(1024, base)).toFixed(decimals);