[Box] Include optional metadata for binary file docs

[navarone-feekery]

Problem Description

There is useful metadata that could be included in the document for a binary file. This can include things like the URL the file was downloaded from, its content type, etc.
What is available would need some light discovery.

Proposed Solution

Add metadata to the doc. This can be done in the stage before extracting content for the _attachment field. This can be done here.

Additional Context

This feature was specifically requested for the Box connector, so I think that is a good place to start. However there's no reason this couldn't be implemented for other Connectors that download files.