Within the New JIRA Issue View, the old attachments coming from the JIRA Server Instance are not displayed with their right names and extensions after migration to Cloud, no preview and the name is a path with an id instead of the <filename.extension>.
The attachment still can be downloaded, and the extension must be added manually to open it.

New JIRA Issue View:

But they are displayed and downloaded normally within the Issue Navigator.

Deleting and Re-uploading the file solve the issue but it can be done manually for all the attachments form the migration.