Thank you for reporting the problem. We have created a GitHub issue to support multiple upload, you can track it here.
In the meantime, possible workarounds:
Quick solution would be to create an intermediate entity File containing a file descriptor and replace the collection of file descriptors in SystemStandard with a collection of Files. That will allow uploading files one by one via File entity editor, and linking Files to SystemStandard via SystemStandard entity (where a multiple selection dropdown will be rendered instead of a file upload component).
Replace FormField with a custom component in your SystemStandard entity editor. You can upload files using this REST API. This will give you the file ids. You can then provide a field value to the Form: