Display inline non-text parts properly

[OJFord]

Inline 'attachments' (i.e. exactly not attachments, but inline, I can't recall the RFC terminology atm) currently render as e.g. 'application/pdf content' (literally) as a placeholder.

It's good that it appears appropriately inline, but should replace that with the same preview/download/thumbnail component used by attachments.

(For now at least - later might be good to have a more inline-optimised, as in better-looking, component for them, but the one already used for attachments should work and be quick to adapt for this purpose.)