Nested attachments only apply to 3 attachment types, these Item attachments are:
When you send an email and attach any of the items above it will generate an email with nested attachments. (An email attached to an email, for example.)
Spanning Backup for Microsoft 365 currently does not support backing up and restoring nested attachments. After restoring an email with nested attachments, only the email itself will be restored and any other standard attachment types: pdf, txt, csv, etc. Item attachments will not be restored.