Rational
It may be useful to display an attachment next to a form filled. For example to encode a supplier invoice, the user may attach the PDF received and fill the form based on the content. With Widget to embed document the attachment form could display such content.
Proposal
We add a new menu entry “preview” to the attachment drop-down to open a Screen next to the form. On tryton we could use a Gtk.HPaned (like for the menu) so the user can resize. On sao we could use the bootstrap col feature to use a good ratio.
The “preview” drop-down entry is used to toggle the visibility and reload of the attachment preview.
There will be a simple navigation toolbar like on form popup with tree view_type.