Setting custom attribute value on file but can't recover it

Hi, I’m running into some troubles.
I have a Concrete\Core\File\Type\Inspector\Inspector that does some processing and then stores values as file attributes. So far so good, it works : the values appear in the File Manager.
However, if I try to get the values back, I get a null. It’s a problem when I use the FileImporter to programmatically add a file to the FileManager and need that attribute to be set for the following processing.
Anyone knows why it happens and how to correct that ? This is driving me crazy, it seems so trivial…

Shortest exemple, in Inspector::inspect

$v = $fv->setAttribute('abc', 123);
// $v->getValueObject() contains the value I just set, all fine !
$fv->getAttribute('abc'); // returns null

I’m a little confused by the example.

If you do:

$fv->setAttribute('abc', 123);

Does that work?