[v3-beta] Read message attachments in backend

[dlqqq]

Problem

In v3, we currently do not read from the message attachments.

Proposed Solution

• The backend should inject the contents of each attached file directly into the prompt.

• @file should add the file as an attachment to the InputModel.

• The FileContextHandler backend class should be replaced with a way to read directly from a list of message attachments. We may want to define this in BaseChatHandler.

Additional context

None known.