add_filter( 'upload_mimes', function( $mimes ) {
$mimes['epub'] = 'application/epub+zip';
return $mimes;
} );
add_filter( 'wp_check_filetype_and_ext', function( $data, $file, $filename, $mimes ) {
$ext = pathinfo( $filename, PATHINFO_EXTENSION );
if ( strtolower( $ext ) === 'epub' ) {
$data['ext'] = 'epub';
$data['type'] = 'application/epub+zip';
}
return $data;
}, 10, 4 );