When providing the list of files in ManagedFile$DescriptorImpl#doFillFileIdItems, if the user doesn't have the permission to access the files, the page might look broken. Providing an empty list with a warning / error message about the permission might provide a better UX.

It is the same of the ManagedFile$DescriptorImpl#doCheckFileId.

This might be easily testable with a user not able to create projects on the Jenkins root level and trying to access the /pipeline-syntax.

This was originally suggested by danielbeck on https://github.com/jenkinsci/config-file-provider-plugin/pull/155#pullrequestreview-812252726. Thank you Daniel.