In order to make a managed_file field permanent I did this in my module configuration form:

<?php  namespace Drupalmy_moduleForm;  use DrupalCoreFormConfigFormBase; use DrupalCoreFormFormStateInterface;  /**  * Class MymoduleSettings.  */ class MymoduleSettings extends ConfigFormBase {    /**    * {@inheritdoc}    */   public function getFormId() {     return 'my_module_settings';   }    /**    * {@inheritdoc}    */   public function buildForm(array $form, FormStateInterface $form_state) {     $config                    = $this->config('my_module.settings');     $form['my_module_title'] = [       '#type'          => 'textfield',       '#title'         => $this->t('Title'),       '#description'   => $this->t("Enter the title"),       '#maxlength'     => 64,       '#size'          => 64,       '#required'      => TRUE,       '#default_value' => $config->get('my_module_title'),     ];     $form['mymodule_icon']   = [       '#type'              => 'managed_file',       '#title'             => $this->t('Icon'),       '#description'       => $this->t("Upload the image file for the icon"),       '#upload_location'   => 'public://',       '#upload_validators' => [         'file_validate_extensions' => ['gif png jpg jpeg'],       ],       '#default_value'     => $config->get('my_module_icon'),     ];     return parent::buildForm($form, $form_state);   }    /**    * {@inheritdoc}    */   public function submitForm(array &$form, FormStateInterface $form_state) {     parent::submitForm($form, $form_state);      $this->config('my_module.settings')       ->set('my_module_title', $form_state->getValue('my_module_title'))       ->set('my_module_icon', $form_state->getValue('my_module_icon'))       ->save();      // First we just grab the file ID for the icon we uploaded, if any.     $icon_field = $form_state->getValue('my_module_icon');     $file_id    = empty($icon_field) ? FALSE : reset($icon_field);      if (!empty($file_id)) {       // Make this a permanent file so that cron doesn't delete it later.       $file         = File::load($file_id);       $file->status = FILE_STATUS_PERMANENT;       $file->save();       $file_usage = Drupal::service('file.usage');       $file_usage->add($file, 'my_module', 'file', $file_id);     }   }    /**    * {@inheritdoc}    */   protected function getEditableConfigNames() {     return [       'my_module.settings',     ];   }  } 

but I’m getting warning to use dependency injection principle in this case:

File::load calls should be avoided in classes, use dependency injection instead 

and

Drupal calls should be avoided in classes, use dependency injection instead 

I looked through Dependency Injection for a Form but still not sure that I really get it.

Can someone explain me what I need to do in this particular case so I can get a better view on this issue and figure it out?