Rational
The Binary field can be displayed with the image widget but sometimes the content that we want to display is not an image but a document like a PDF.
Proposal
We add a new type document of widget for <form/>. It tries to display the content of a Binary field without any edition.
For sao we could use the <object/> tag which support many types (depending of the browser).
For tryton we could use the Evince View (which is not very well documented).
This widget would be used for the attachment instead of the image.