I have this form 
created by this code:
public function buildForm(array $ form, FormStateInterface $ form_state, $ contractId = NULL) { some initialisation code... $ database = \Drupal::database(); // From database to populate $ form['designation'] $ query_am = $ database->select('member', 'am'); $ query_am->fields('am', ['id', 'designation']); $ query_am->condition('status', [2, 3, 4], 'IN') ->orderBy('designation', 'ASC'); $ aMembers = $ query_am->execute()->fetchAllKeyed(); $ aMembers = ["0" => ""] + $ aMembers; $ memberId = $ form_state->getValue('designation'); if ($ memberId) { $ query = $ database->select('contract_subscription', 'cs') ->fields('cs') ->condition('contract_id', $ contractId) ->condition('member_id', $ memberId); $ subscription = $ query->execute()->fetchAssoc(); } $ wrapper = 'formWrapper'; $ form['#prefix'] = '<div id="' . $ wrapper . '">'; $ form['#suffix'] = '</div>'; $ form['designation'] = [ '#type' => 'select', '#options' => $ aMembers, '#title' => $ this->t('Member'), '#required' => TRUE, '#default_value' => $ memberId, '#ajax' => [ 'callback' => '::memberCallback', 'wrapper' => 'theSubscription', ], ]; $ form['theSubscription'] = [ '#type' => 'container', '#attributes' => ['id' => 'theSubscription'], '#quantities' => $ iNumberOfQuantities, ]; $ value = $ subscription['quantity01']; $ form['theSubscription']['quantity01'] = [ '#type' => 'number', '#title' => $ aContractTypeHeader[$ i - 1], '#min' => 0.00, '#max' => 99.95, '#step' => 0.05, '#value' => $ value, ]; } $ form['theSubscription']['comment'] = [ '#type' => 'textarea', '#title' => t('Comment'), '#rows' => 1, '#value' => $ subscription['comment'], ]; $ fileId = (int) $ subscription['file__target_id']; $ form['theSubscription']['file'] = [ '#type' => 'managed_file', '#description' => $ this->t('Limited to @size MB.', ['@size' => $ upload_max_inMB]) . '<br>' . $ this->t('Allowed types: pdf.'), '#upload_location' => 'private://contracts/subscriptions/', '#upload_validators' => [ 'file_validate_extensions' => ['pdf'], 'file_validate_size' => [$ upload_max], ], '#default_value' => [$ fileId], ]; if ($ sContractIsOpenForSubscription) { $ form['save'] = [ '#type' => 'submit', '#name' => 'save', '#value' => $ this->t('Save'), '#ajax' => [ 'wrapper' => $ wrapper, 'callback' => '::ajaxSave', ], ]; } $ form['leave'] = [ '#type' => 'submit', '#name' => 'leave', '#value' => $ this->t('Leave'), ]; $ form['#attached']['library'][] = 'amap/amap'; return $ form; } public function memberCallback($ form, FormStateInterface $ form_state) { return $ form['theSubscription']; } When I select an item in the select box, Ajax memberCallbackis called, $ database->select('contract_subscription', 'cs') is executed and the form is updated with the values retrieved from the database.
That works for all the fields except for the file which is never displayed (only the default empty managed_file field shows up): 
I wanted to check and I executed the form with $ memberId = 64; hardcoded to force a first display without using Ajax and it works! 
What is wrong with my code?