I’m writing a block plugin that uses the form API to allow users to edit its settings, and then output them into a template.

I’m currently using

<?php  /**  * @file  * Contains Drupalmy_pluginsPluginBlockInfoWithSideImage.  */  namespace Drupalmy_pluginsPluginBlock;  use DrupalCoreBlockBlockBase; use DrupalCoreFormFormStateInterface;  /**  * @Block(  * id = "infowithsideimage",  * admin_label = @Translation("Info block beside an image")  * )  */  class InfoWithSideImage extends BlockBase {     public function blockForm($form, FormStateInterface $form_state) {         $form['image'] = array(             '#type'          => 'file',             '#title'         => $this->t('Image'),             '#default_value' => $this->configuration['image_submit'],         );         $form['test'] = array(             '#type' => 'textfield',             '#title' => $this->t('Test text field'),             '#default_value' => $this->configuration['test_submit'],                         );         return $form;     }     public function blockSubmit($form, FormStateInterface $form_state) {         $this->configuration['image_submit'] = $form_state->getValue('image');         $this->configuration['test_submit'] = $form_state->getValue('test');     }      public function build() {         return array(             '#theme' => 'infowithsideimage',             '#image' => $this->configuration['image_submit'],             '#test' => $this->configuration['test_submit'],         );     } } 

The text field works fine, but whenever I try to submit with the file field I get the following error:

A fatal error occurred: Serialization of SymfonyComponentHttpFoundationFileUploadedFile is not allowed

Is there something I’m missing? Is it possible to use the form API to allow users to upload files?