Error on combined upload

Problem/Motivation

I can not upload any Media with the Combined Upload enabled.

Error occurs on 2.2.0-alpha-2 on Drupal 10.4.2
Error does not occur on 2.1.0-beta5 on Drupal 10.4.2
Not tested on Drupal 11 yet.

Steps to reproduce

1. In /admin/config/media/media_directories, enable 'Combined Upload'. Wether all or only some or one Media Type gets activated, does not matter.

2. Go to Media-Directory-Browser, be it via Media-Directory-Embed-Button in a Ckeditor5-editor, or via /admin/content/browser

3. Choose any Directory. Wether its Root, Root with all files shown or any subdirectory does not matter.

4. Click 'Add Media'. The Media-Upload-Modal presents.

5. In Field 'Select Fields', click Browse

6. In the file-browser, choose any number, any type of file. Tested with one and multiple images (png and jpg), with one and multiple pdfs, with one image and one pdf.

7. On clicking 'Choose' on the file-browser, an error occurs.
- The Media-Upload-Modal is still open, but below, on the /admin/content/browser-page there is a red-on-black error-message "Add-Media-Modal is still here, but in Browser-Page below, we get a red-on-black error-message 'Oops, something went wrong. Check your browser's developer console for more details'. "
- The browser's developer console says:

Object { message: "
An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /en/admin/media_directories/media/add?media_type=combined_upload&target_bundles%5Baudio%5D=audio&target_bundles%5Bdocument%5D=document&target_bundles%5Bimage%5D=image&target_bundles%5Bremote_video%5D=remote_video&target_bundles%5Bvideo%5D=video&active_directory=-1&cardinality=-1&selection_mode=keep&ajax_form=1
StatusText: 500 Service unavailable (with message)
ResponseText: The website encountered an unexpected error. Try again later.TypeError: array_merge(): Argument #2 must be of type array, Symfony\Component\Validator\ConstraintViolationList given in array_merge() (line 55 of core/modules/file/src/Validation/FileValidator.php). Drupal\file\Validation\FileValidator->validate() (Line: 285)
Drupal\file\Upload\FileUploadHandler->handleFileUpload() (Line: 660)
file_save_upload() (Line: 536)
_file_save_upload_from_form() (Line: 1007)
file_managed_file_save_upload() (Line: 76)
Drupal\file\Element\ManagedFile::valueCallback()
call_user_func_array() (Line: 1264)
Drupal\Core\Form\FormBuilder->handleInputElement() (Line: 1004)
Drupal\Core\Form\FormBuilder->doBuildForm() (Line: 1074)
Drupal\Core\Form\FormBuilder->doBuildForm() (Line: 1074)
Drupal\Core\Form\FormBuilder->doBuildForm() (Line: 579)
Drupal\Core\Form\FormBuilder->processForm() (Line: 326)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 240)
media_directories_ui_preprocess_media_directories_add()
call_user_func_array() (Line: 261)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 153)
Drupal\Core\Render\Renderer->Drupal\Core\Render\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 152)
Drupal\Core\Render\Renderer->renderRoot() (Line: 36)
Drupal\Core\Ajax\OpenDialogCommand->getRenderedContent() (Line: 148)
Drupal\Core\Ajax\OpenDialogCommand->render() (Line: 43)
Drupal\Core\Ajax\AjaxResponse->addCommand() (Line: 403)
Drupal\media_directories_ui\Controller\MediaDirectoriesController->mediaAdd()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 116)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 90)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\
egotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)
", name: "AjaxError", stack: "@http://URL_REDACTED/core/misc/ajax.js?v=10.4.2:196:32
@http://OUR_URL/core/misc/ajax.js?v=10.4.2:1935:3
" }

- 'drush ws' says:

  ID       Date           Type   Severity   Message
 -------- -------------- ------ ---------- ----------------------------------------------------------------------------
  689316   07/Feb 10:30   php    Fehler     TypeError: array_merge(): Argument #2 must be of type array,
                                            Symfony\Component\Validator\ConstraintViolationList given in array_merge()
                                            (line 55 of
                                            PATH_REDACTED/src/drupal/cor
                                            e/modules/file/src/Validation/FileValidator.php).

Proposed resolution

Workaround: Disable 'Combined Upload' in /admin/config/media/media_directories. Without combined upload, uploading pdfs and images works as expected.

Remaining tasks

User interface changes

API changes

Data model changes