[Vondry]

Problem

When an image or imagelist field has no extensions set, the defaults are built by intersecting ['gif', 'jpg', 'jpeg', 'png', 'svg', 'avif', 'webp'] with accept_file_types. Collection::intersect() keeps the original keys, so disallowing a type in the middle of that list leaves a gap.

For example, with svg removed from accept_file_types (a common hardening step) but avif and webp still allowed, the field definition ends up with:

{"0":"gif","1":"jpg","2":"jpeg","3":"png","5":"avif","6":"webp"}

That's passed to the editor as a JSON object instead of an array, so clicking "Add new image" fails with:

TypeError: this.extensions.map is not a function
    at a.acceptedExtensions (bolt.js)

The existing tests didn't catch this because the shipped config.yaml allows neither svg, avif nor webp, so the intersection only trims the end of the list and the keys stay sequential.

Fix

• ContentTypesParser: reindex extensions with array_values() where file, filelist, image and imagelist fields are already normalised to an array. This covers the defaults as well as any extensions set in contenttypes.yaml.
• file, filelist and imagelist templates: default extensions to [] instead of '', like image.html.twig already does, so an empty value is also passed as an array.

Tests

Added ContentTypesParserTest::testUploadFieldExtensionsAreLists with a fixture holding all four upload field types without extensions. It sets accept_file_types without svg and asserts that each field's extensions are a list that serialises as a JSON array. It fails without the fix (keys jump from 3 to 5) and passes with it.