Document modern image formats in forms and validation

docs/en/core-libraries/validation.md

@@ -675,6 +675,21 @@ Core rules that take additional parameters should have an array for the
 `rule` key that contains the rule as the first element, and the additional
 parameters as the remaining parameters.
 
+### File Extensions
+
+`Validation::extension()` accepts `gif`, `jpeg`, `png`, `jpg`, `jxl`, `webp`,
+`avif` and `avifs` by default. Supply an explicit list to restrict extensions
+for a particular upload:
+
+```php
+$validator->add('photo', 'extension', [
+    'rule' => ['extension', ['jpg', 'jpeg', 'png']],
+]);
+```
+
+The rule checks the filename extension, not the file's contents. Use a MIME
+type rule as well when you need to check the uploaded content.
+
 ### IP and Range Validation
 
 You can validate that a value is a valid IP address or an IP range (subnet)

docs/en/views/helpers/form.md

@@ -2020,7 +2020,8 @@ You could then use JavaScript to set the `onvalid` and `oninvalid` events as you
 
 Creates an `input` element of `submit` type, with `$caption` as value.
 If the supplied `$caption` is a URL pointing to an image (i.e. if the string
-contains '://' or contains any of the extensions '.jpg, .jpe, .jpeg, .gif'),
+contains '://' or ends in `.jpg`, `.jpe`, `.jpeg`, `.jxl`, `.gif`, `.png`,
+`.webp`, `.ico`, `.avif` or `.avifs`),
 an image submit button will be generated, using the specified image if it
 exists. If the first character is '/' then the image path is relative to
 *webroot*, else if the first character is not '/' then the image path is