2.5 inlist clean

Resolves https://cakephp.lighthouseapp.com/projects/42648/tickets/3521-case-insensitive-validation-inlist-option

Also fixes the issue that $strict would return too many false positives by removing it all-together and putting the focus on what actually needs a setting: case sensitivity.

Last but not least clarifies that mimetype validation/check is case sensitive.