Behavior Changes

Guessing the file extension from the MIME type has been changed if the proposed extension is not valid. Previously, the guessing will early terminate and return null. Now, if a proposed extension is given and is invalid, the MIME guessing will continue checking using the mapping of extension to MIME types.
If there is a cookie with a prefixed name and a cookie with the same name without a prefix, the previous get_cookie() had the tricky behavior of returning the cookie without the prefix. Now the behavior has been fixed as a bug, and has been changed. See :ref:Upgrading <upgrade-421-get_cookie> for details.

Bugs Fixed

See the repo's CHANGELOG_4.2.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/changelogs/CHANGELOG_4.2.md>_ for a complete list of bugs fixed.