Text tries to open EPub files while it cannot display them correctly #787

Describe the bug
Text tries to open Epub files while it cannot display them correctly. The user only gets garbled text shown.

To Reproduce

1. Open an Epub file in Nextcloud

Expected behavior
Stop opening Epub files or add support for them. There is a viewer for Epub files out there already: https://github.com/e-alfred/epubreader Onlyoffice supports viewing them too.

Screenshots