Always show object content, regardless of object type, if mediaType is supported

[Oblomov]

Pitch

Currently, Mastodon separates ActivityPub objects between “supported” and “converted”, based on object type. Converted objects will only display a title, spoiler text and URL. This is the case even if the object has a content that could be handled no differently than the Note type.

A better approach would be to map object properties to the internal model whenever that makes sense, regardless of the object type. This would mean mapping the title & summary to the spoiler text, the (sanitized) content to the text (if supported, i.e. if its mediaType is the default text/html or other supported format), etc.

Motivation

The choice to reduce non-supported types to “title + link” even when content or other compatible attributes are present has no apparent motivation, and leads to issues ranging from the one described in #8892 or #19357 to poor user experience both for Mastodon users and for users of other federate platforms that wish to be more accessible to Mastodon users.

Classic example is the ActivityPub plugin for WordPress recommending to use Note instead of Article as AP object type, even though Article would be the more sensible choice, because otherwise Mastodon followers won't be able to see the post content. Arguably, PixelFed's choice to use Note as object type is due to the same reasons.

I know that at least for Articles the official rationale behind the choice is that Mastodon is intended for short messages only, but the fact that Mastodon accepts the exact same content when it's presented as Note instead of an Article (or anything else), the rationale seems ultimately pretty weak, and the practical effect is that other platforms end up abusing the Note type to improve the Mastodon users' experience and avoid being relegated to “title and link” on the dominant platform.

So, if there isn't a stronger rationale than “Mastodon is supposed to be for short messages only” (that ship has long sailed), I think it would be better for the entire ecosystem if object attributes were mapped to equivalent status attributes whenever possible, independently from the object type.