To add file upload validation for a custom form field in the Adobe Commerce admin panel, which should restrict the file type to .pdf and limit the file size to 2 MB, the recommended approach is to include the validation parameters directly within the node in the form’s XML configuration. This ensures that the validation occurs on the client-side as well as server-side, providing immediate feedback to users before submission.

Option C is correct for the following reasons:

Adding Validation Inside <settings>:By placing the <allowedExtensions> and tags within the node of the XML configuration, the system will enforce these restrictions directly within the form component. This method leverages Magento’s built-in support for validation settings, which ensures that only files matching the specified criteria (.pdf files of 2 MB or smaller) are accepted by the form.

Explanation: Magento UI components allow for client-side validation through the node, where attributes such as allowedExtensions and maxFileSize are used to control file upload constraints. This approach not only validates the file size and type but also integrates seamlessly with Magento’s front-end validation mechanisms.

References: Magento’s documentation on UI components highlights how to enforce file type and size restrictions through XML configurations within , making it the standard and most efficient solution for this task.

Alternatives and Limitations:

Option A: Adding validation within the controller (UploadPdf) can provide additional server-side validation, but this does not prevent the user from selecting invalid files in the first place. Server-side validation alone lacks the user experience enhancement provided by client-side feedback.

Option B: Configuring a virtual type in di.xml for validation (e.g., setting allowedExtensions and maxFileSize within a custom uploader model) is effective for backend processing but is not as straightforward for direct form validation within the admin UI. It complicates the implementation by requiring custom backend logic where native XML validation can suffice.

Option C provides a straightforward, maintainable, and user-friendly way to implement file validation directly in the form configuration. It reduces the need for custom controller or model logic and leverages Magento’s built-in form handling capabilities.