Comment 43 for bug 39136

Created attachment 9404704
Bug 196078 - Part 2: Support displaying arbitrary text/* MIME types as plain text, r=smaug!

This patch refactors how we check for text formats when deciding how to handle
resources, such that more text MIME types will be rendered in-browser, rather
than downloaded.

This change requires us to move more away from using the Gecko-Content-Viewers
category in the category manager for this decision, as we need to handle an
unlimited number of MIME types behind the scenes.

Support for Gecko-Content-Viewers was left in for both the in-tree use for
application/http-index-format and dynamically determining whether image/avif
and image/jxl are supported, as well as for the message/rfc822 type used by
Thunderbird.

Depends on D212077