[MM-28923] Fix errcheck issues in server/channels/app/file.go (#30878)

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -90,7 +90,6 @@ issues:
         channels/api4/websocket_test.go|\
         channels/app/bot_test.go|\
         channels/app/brand.go|\
-        channels/app/file.go|\
         channels/app/file_test.go|\
         channels/app/helper_test.go|\
         channels/app/permissions_test.go|\