UploadField doesn't validate allowed extensions with "choose existing" option

[mark-a-j-adriano]

Module version(s) affected

2.1.3

Description

Allowed extensions validation for UploadField does not kick in when using the "choose existing" option

https://github.com/silverstripe/silverstripe-asset-admin/assets/11882563/9f2a02f8-ed41-4552-8359-684d4e418304

How to reproduce

1. Add an upload field like this to a form:

   /** @var UploadField $logo */
   $logo = UploadField::create('Logo', 'Logo');
   $logo->setAllowedExtensions(['png', 'svg'])
       ->setAllowedMaxFileNumber(1)
       ->setUploadEnabled(false)
       ->setDescription('Accepts PNG or SVG image format.');

2. Add a file (not png or svg) to the field using the "choose existing" option
3. Note that the file is added to the field without raising a validation error.

NOTE the setAllowedMaxFileNumber is probably unrelated, and setUploadEnabled is not necessary.

Possible Solution

No response

Additional Context

No response

Validations

• [X] Check that there isn't already an issue that reports the same bug
• [X] Double check that your reproduction steps work in a fresh installation of silverstripe/installer (with any code examples you've provided)

Notes

• That validation is currently handled by the upload validator - which is for validating uploaded files, not existing files
• It would make sense for the allowed extensions validation to be triggered for existing files. It's not uncommon for business logic to be "this relation should only be files of this type" (e.g. videos only, or documents only, etc)
• This is a fairly substantial change in logic, it should probably be introduced in a minor release rather than a patch so people can be alerted to this change via the changelog.

Acceptance criteria

• User is blocked from selecting pre-existing files that do not match the allowed extensions.
• All file validation rules are honoured when selecting a pre-existing file
• The Browser upload file dialog stops you from uploading files that do not have one of the allowed extensions. (What should happen we have the default validation rules? Should all the file upload input include all the CMS allowed types?)

UX questions

• What happens if you select a file that does not meet the validation criteria? How do we communicate to the user that they cannot select this file?