Hi Juan,
in my courses I 'm using some epub files for my students. The epub files are shown in a folder without their epub icon. In Moodle they have an icon with a green "e".
Moodle mobile says that it is able to download epub files inside the app ... this should be wrong. If Moodle mobile will be able to download the epub files then the files should be sent to another app. On my iPad an epub can be shown with the iBooks app but also with the Book Creator or the Creative Book Builder.
Best regards, Ralf