is form enctype "application/json" available?

The W3C publishes many drafts and proposals which are then discussed within the community at large. If a draft makes it to the stage where it's generally considered useful, browser vendors will/may start implementing it. The draft then typically advances to a "recommendation" stage, meaning the W3C officially recommends that browsers implement the technology as specified; but of course they can't twist anyone's arm to actually do so.

Each document will say at its top what its current status is, and http://www.w3.org/TR/ lists all current documents and their status. The one you picked is listed as "obsolete" and "retired" on that page and has a ginormous banner at its top saying:

Beware. This specification is no longer in active maintenance and the HTML Working Group does not intend to maintain it further.

So, no, probably no browser is currently implementing it.

To track the real-world availability of a feature you need to look to 3rd party resources like https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form#attr-enctype and http://caniuse.com.

FYI.. there's a hacky way to mimic this functionality using javascript. Seem my library here: https://github.com/keithhackbarth/submitAsJSON