[O] Agenda file is not in `org-mode'

[Marcelo de Moraes Serpa]

Hey guys,

When trying to show up the archived items in the agenda view, I'm getting
this message:

"Agenda file /Users/fullofcaffeine/org/gtd/gtd.org_archive is not in
`org-mode'"

To fix it, I have to manually open gtd.org_archive, and "M-x org-mode" it.

Any ideas on why this happens?

Thanks in advance,

Marcelo.