Changes in API behavior:

* Introduce a new endpoint `POST /api/v3/file_links`.
* The request/response signature is similar to `POST /api/v3/work_packages/{id}/file_links`, but there is no work\_package known during the request time. Therefore, we create a file\_link without a container and respond without a container link.
* Implementation notes:
* Begin of the insertionToEnd of the insertion Begin of the deletion toEnd of the deletion be able to create file\_links without a known container we have to remove not null constraints from the `file_links.container_id` and `file_links.container_type`  database columns.
* Begin of the insertionWeEnd of the insertion Begin of the deletion weEnd of the deletion have to cover a case when file\_links Begin of the insertionwereEnd of the insertion Begin of the deletion has beenEnd of the deletion created, but Begin of the insertiontheEnd of the insertion work\_package create form has never been Begin of the insertionsubmitted.End of the insertion Begin of the deletion confirmed.End of the deletion To do it we can introduce a Begin of the insertion`Cron::CronJob`End of the insertion Begin of the deletion cron jobEnd of the deletion for cleaning up.
`app/workers/attachments/cleanup_uncontainered_job.rb` - the one for handle a similar case for Begin of the insertionattachments.End of the insertion Begin of the deletion attachments. End of the deletion
* Modify `POST` [`https://openproject.local/api/v3/work_packages`](https://openproject.local/api/v3/work_packages)
* It should accept a list of file\_links by analogy to attachments. And respond with them Begin of the insertionafterwards.End of the insertion Begin of the deletion afterwards. End of the deletion
* Implementation note: the file\_links have to be bound to a container (work\_package in this case). The logic for attachments bindings is located in `app/services/attachments/set_replacements.rb`