Extended Tags don't show at all if multiple files are selected [#16924]

[cdemeillon]

Unlike other tag values, Extended tags that have the same value don't show up at all when multiple mp3 files are selected.
As soon as you select more than one track, all Extended Tags disappear and do not allow viewing or editing them.

[Peke]

HI,
THX for reporting added as https://www.ventismedia.com/mantis/view.php?id=16924