What hook gets called at the formation of a form? (hook_form_alter called for adjusting a form)

Question

what hook gets called at the formation of a form.

I'm trying to add some additional mime types to form with id file_entity_add_upload but my hook_form_alter is getting called after these are validated, so attempts to upload them fails.

Update - added screenshot to illustrate the problem

Update 2 the mime types list in file_entity_get_upload_validators function in file_entity.pages.inc dictates what can be uploaded - see this screenshot

Specifically, in, file_entity_get_upload_validators, the line that populates the valid mime types with hard-coded values is (my screenshot didn't completely show the line): $validators['file_validate_extensions'] = array(variable_get('file_entity_default_allowed_extensions', 'jpg jpeg gif png txt doc docx xls xlsx pdf ppt pptx pps ppsx odt ods odp'));

Update 3

It would appear that the actual checking of the mime type happens in core module "file", modules/file/file.js : if (!acceptableMatch.test(this.value)) { var error = Drupal.t("The selected file %filename cannot be uploaded. Only files with the following extensions are allowed: %extensions.", { I searched the error message shown in following screenshot in the code and file.js was a result - which looks like where the actual validation is taking place.

New error with Andy's answer - maybe it's me will look into it a little later.

Raised as an issue here: https://www.drupal.org/node/2295671

Answer 1

file_entity_add_upload is a three step form for uploading files when you have fieldable files.

function file_entity_add_upload($form, &$form_state, array $options = array()) {
  $step = (isset($form_state['step']) && in_array($form_state['step'], array(1, 2, 3))) ? $form_state['step'] : 1;
  $form['#step'] = $step;
  switch ($step) {
    case 1:
      return file_entity_add_upload_step_upload($form, $form_state, $options);
    case 2:
      return file_entity_add_upload_step_filetype($form, $form_state, $options);
    case 3:
      return file_entity_add_upload_step_fields($form, $form_state, $options);
  }
}

1. Upload the file (validates extensions);
2. Choose the file bundle (based on bundle mimetype associations);
3. Fill out fields attached to that bundle.

As the extensions are validated in step 1, examine file_entity_add_upload_step_upload(). Of particular interest is the actual managed file element.

$form['upload'] = array(
  '#type' => 'managed_file', 
  '#title' => t('Upload a new file'), 
  '#upload_location' => file_entity_upload_destination_uri($options), 
  '#upload_validators' => file_entity_get_upload_validators($options), 
  '#progress_indicator' => 'bar', 
  '#required' => TRUE, 
  '#pre_render' => array('file_managed_file_pre_render', 'file_entity_upload_validators_pre_render'), 
  '#default_value' => isset($form_state['storage']['upload']) ? $form_state['storage']['upload'] : NULL,
);

The validators are added using a non-standard property #upload_validators to set the file validation callbacks and their arguments. In the form alter we can completely overwrite #upload_validators or just modify it, and that will affect the file's validation as you'd expect. However if you look in file_entity_get_upload_validators():

if (!empty($options['file_extensions'])) {
  $validators['file_validate_extensions'] = array($options['file_extensions']);
}
else {
  $validators['file_validate_extensions'] = array(variable_get('file_entity_default_allowed_extensions', 'jpg jpeg gif png txt doc docx xls xlsx pdf ppt pptx pps ppsx odt ods odp'));
}

You can see that the variable file_entity_default_allowed_extensions is used to specify which extensions to use by default (unless it's overridden by $options).

The managed_file element itself passes the extensions to JS for client-side validation in file_managed_file_process():

// Add the extension list to the page as JavaScript settings.
if (isset($element['#upload_validators']['file_validate_extensions'][0])) {
  $extension_list = implode(',', array_filter(explode(' ', $element['#upload_validators']['file_validate_extensions'][0])));
  $element['upload']['#attached']['js'] = array(
    array(
      'type' => 'setting',
      'data' => array('file' => array('elements' => array('#' . $element['#id'] . '-upload' => $extension_list))),
    ),
  );
}

As you can see from pcambra's neat illustration, processing happens after form alters are called, so any modifications you make will affect both client-side and server-side validation.

So to make a global change modify the variable file_entity_default_allowed_extensions: I'd suggest using drush or a hook_update_N().

To modify a subset of forms you can use a form alter. Here's an example:

/**
 * Implements hook_form_FORM_ID_alter() for file_entity_add_upload.
 *
 * Modifies the allowed extensions.
 */
function MYMODULE_form_file_entity_add_upload_alter(&$form, &$form_state) {

  // Check this is the file selection step.
  if ($form['#step'] != 1) {
    return;
  }

  $form['upload']['#upload_validators']['file_validate_extensions'][0] = 'png bmp';
}

Answer 2

There's this brilliant image on the Form API Internal Workflow in drupal.org, you can see the build process and structure. You've got callbacks such as #process and #after_build that could be useful for your use case.