Wrong content type returned for AJAX upload to async-upload.php

[dvershinin]

When user uploads, e.g. featured image to post, the upload endpoint is /wp-admin/async-upload.php.
Both upload request and server response are in JSON format.

However, despite returning valid JSON data, the async-upload.php sends this header:

Content-Type: text/html; charset=UTF-8

Wherein it should be:

Content-Type: application/json; charset=UTF-8

Content type mismatch may be an issue to some server side security software...

[SergeyBiryukov]

Hi @dvershinin, welcome to WordPress Trac!

Thanks for the report, we're already tracking this issue in #40323.