See CollectionsController#keep_web_url.

From #5824 note 25:

• If keep_web_download_url is set, disposition=attachment links (i.e., downloads) are routed to the given URL (typically https://download.uuid_prefix.arvadosapi.com/c=%{uuid_or_pdh}). This is expected to match keep-web's -attachment-only-host argument.
• If keep_web_download_url is set and keep_web_url is not set, preview is disabled and all preview/download links are routed to keep_web_download_url.
• If both are set, but keep_web_url isn't a wildcard (one-vhost-per-collection) form and trust_all_content isn't true, showing inline content from a non-public non-shareable-link collection will result in a download instead of preview.
• If keep_web_download_url is not set, and keep_web_url isn't a wildcard form, and trust_all_content isn't set, showing inline content from a non-public non-shareable-link collection will produce an error: with that configuration, we can't authenticate safely without creating XSS problems.

Don't render file preview buttons in cases 2-4.