When you reply to or forward a conversation that contains attached files, Outpost uses smart handling of the attachments.
Attachments drop from replies
When you reply to any conversation that originally contained an attachment, the attachment file will not be included in your reply. Outpost is operating on the assumption here that the person you're replying to has already seen the attachment:
If you are adding more recipients to your reply and they need to see the attachment, compose a forward instead, so the attachment remains on your message.
Attachments remain on forwards
When you forward a conversation, any attachments will remain attached to your forwarded message. Outpost is operating on the assumption here that since you're sending the conversation on to a new recipient, they need to see the attachment: