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I'm on Drupal 8. I have created a new media bundle (permissions_json) which can be created from /media/add/permissions_json, where administrator user can upload specifically formatted JSON files, and I can upload to it all fine and dandy.

I'd like to validate the file uploaded and rename it. The problem is that I can't find a way to hook into the form. Implementing hook_form_FORM_ID_alter() with gp_mcs_media_permissions_json_add_form_alter() or gp_mcs_form_media_permissions_json_add_form_alter() doesn't have any effect. Even the following implementation didn't have any effect.

function gp_mcs_form_alter(&$form, &$form_state, $form_id) {
  dpm($form);
}

If I put a simple exit() on the module's page stops the website, so the file is accessible. I just can't figure out how to add validation to this add media form.

How can I alter the form to add a new validation handler?

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The form ID for an entity is a string that is usually returned from EntityForm::getFormId(), if the class used for the form doesn't override that method.

This means that, by default, the form ID follows the <entity>_<bundle>_<operation>_form schema, where the <operation> part is omitted, when operation is default.
Since the Media module doesn't have a class that overrides EntityForm::getFormId(), the correct ID for the form that creates a new Media entity whose bundle is permissions_json is media_permissions_json_add. This means that a module whose machine name is gp_mcs can alter that form by implementing hook_form_FORM_ID_alter() with a function named gp_mcs_form_media_permissions_json_add_alter. (Note that the form ID is between form and alter.)

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