Add Asset Template does not use api_assets config

[jlayt]

The allowed Asset media types and file extensions are configured using api_assets in module.config.php and local.config.php. AssetAdapter uses these settings in server-side validation, but the client-side validation in application/view/omeka/asset/add-form.phtml does not use the settings and hard codes the allowed extensions instead. The add-form should be using the config.