Attachment upload and download

[KevM]

Attachments are a PITA in Clarify. We need a standard endpoint that accepts attachment uploads.

The web end point should take a file upload and an object to which it should be attached.

The backend storage of this attachment should be modular and default the to the standard Clarify mechanism of storing it to a file path written to the doc_path record.

Alternative backends that store attachments "in the cloud" would be fantastic
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