Make compatible with Drupal 9/10

The problem looked simple at the first glance - not initialised extensions property, but when I fixed this and hit save it told me that config_export is missing on the ConfigEntity, which is this https://www.drupal.org/node/2949023 → change record for Drupal 8.6.

Than it thrown TypeError: Drupal\file_downloader\DownloadOptionPluginBase::addContextAssignmentElement(): Argument #1 ($plugin) must be of type Drupal\Core\Plugin\ContextAwarePluginInterface, Drupal\file_downloader\Plugin\DownloadOption\OriginalFile given, called in /app/web/modules/contrib/file_downloader/src/DownloadOptionPluginBase.php on line 118 in Drupal\file_downloader\DownloadOptionPluginBase->addContextAssignmentElement() (line 37 of /app/web/core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php) at me on the edit screen...

Fixed that , and than it was time of Error: Call to undefined method Drupal\Core\File\FileSystem::getScheme() in Drupal\file_downloader\DownloadOptionPluginBase->deliver() (line 71 of modules/contrib/file_downloader/src/DownloadOptionPluginBase.php). , which is this https://www.drupal.org/node/3035273 → change record for Drupal 8.8...

It looks like this module wasn't tested with Drupal 9 at all...

Will run an full compatibility check a bit later and do some more testing.

What was done/changed:

• ContextAwarePluginInterface interface added to the list of implemented by the DownloadOptionPluginBase class (was implemented via ContextAwarePluginTrait)
• replaced file system service with stream wrapper manager services in DownloadOptionPluginBase and Drupal\file_downloader\Plugin\DownloadOption\ImageStyle classes
• Extended config entity type annotation with the config_export property , listing all the properties from the configuration schema
• set default value for the list of allowed extensions for the download option

Clicking the "Add Download option config" link the /admin/config/media/download_options/add page results in : issue resolved. But when we tried to add any download option then issue are coming.

 The website encountered an unexpected error. Please try again later.

Drupal\Core\Config\Schema\SchemaIncompleteException: Entity type 'Drupal\file_downloader\Entity\DownloadOptionConfig' is missing 'config_export' definition in its annotation in Drupal\Core\Config\Entity\ConfigEntityBase->toArray() (line 253 of core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php).
Drupal\Core\Config\Entity\ConfigEntityStorage->mapToStorageRecord(Object) (Line: 273)
Drupal\Core\Config\Entity\ConfigEntityStorage->doSave('small', Object) (Line: 520)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 253)
Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object) (Line: 339)
Drupal\Core\Entity\EntityBase->save() (Line: 608)
Drupal\Core\Config\Entity\ConfigEntityBase->save() (Line: 152)
Drupal\file_downloader\Form\DownloadOptionConfigForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 595)
Drupal\Core\Form\FormBuilder->processForm('download_option_config_add_form', Array, Object) (Line: 323)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19) 

Steps to reproduce

1. Install the latest 2.0.1version of this module
2. Enable it
3. Navigate to the /admin/config/media/download_options/add page

when tried to add any option issues are coming

Have you applied the patch correctly? Have you cleared the cache after that?

I've already tested it on two different code bases - works as expected.

Thanks for the patch. The patch was applied successfully and is working as expected after clearing the cache.
Moving RTBC.

Thanks @RoSk0, After clear cache working as expected. Please review screenshot for more reference,hence created MR.

Rerolling against 3.0.0

The same issue is present on the 3.0.0 branch - I am rather disheartened that a new 3.0.0 version has been released with this issue still present and no engagement from the maintainers on this issue despite it being RTBC'd for over 6 months.