Some corrections to 9.2.5 JSON Feed to JF2 conversion

When reading JSON Feed, keys are in snake case not kebab case, so:

> Rename "mime-type" to "content-type"

to "mime_type"

Also, what to do with audio attachments? If the "content-type" field begins with "audio/", add the item to the "audio" property.

Locate the JSON Feed to JF2 conversion entry point and inspect how snake_case keys and attachments are handled. Verify the conversion uses "mime_type" and places audio attachments in the "audio" property when the content type begins with "audio/"; no file or test is named in the issue.

Written by the indexing model from the issue text.