Set value of file field using entity_metadata_wrapper non working
Fixed! Thanks to this post Save a new value with entity_metadata_wrapper to an entity field which is an array

$file = file_load(8172);
$node = entity_load('sys_ind', array(13718));
$wrapper = entity_metadata_wrapper('sys_ind', '13718');

$wrapper-&gt;field_allegati[NULL]-&gt;file-&gt;set( $file ); # This worked

$wrapper-&gt;save();


so the fix is using NULL (at least in multivalue field like mine is), not 0 or any other index nor [].