The attachment email workflow sends secure, expirable links. It does not attach the underlying files directly to the email.
You need both Download Attachments and Send Case Emails. The case must allow the action.
For an external link, confirm that the recipient can access the source file through its provider. Receiving a CaseWyze link does not grant Google Drive, OneDrive, or SharePoint access.
Check whether the secure link expired, was revoked, reached its access limit, or requires a password or specific email. For a cloud file, also review the source URL and provider permissions.