Bug overview description.

Can't add a post to a set through the API described here.

What part(s) of the site page(s) are affected?

/set/add_post.xml

What is the expected behavior?

Post is added to the set

What actual behavior is given instead?

Unexpected error but that's probably because it was a GET request. POST request with login and password_hash supplied returns a 403 Forbidden nginx page.

Time of incident (if applicable).

Today

Can you reproduce the bug every time?

Yes

What steps did you take to replicate this bug?

Use any method to send a POST request to https://e621.net/set/add_post.xml with valid set_id, post_id, login, and password_hash parameters.

Errors or other messages returned (if any).

403 Forbidden - nginx/1.12.1 (POST request)

OR

<response>
  <success type="boolean">
    false
  </success>
  <reason>
    Unexpected error (140b20728747e965a0c76abe37b8220e)
  </reason>
</response>

(GET)

Confirmed, there appears to be no controller for json/xml requests.
Imported for fixing.