Handle External URL Schemes Like ftp:// in File URL Fields Without Errors

Problem/Motivation

When attempting to use an external URL (e.g., ftp://example.com/file.txt) as the value for a file_url field, the following error is triggered:

Drupal\Core\File\Exception\InvalidStreamWrapperException in Drupal\Core\File\FileUrlGenerator->doGenerateString() (line 105 of core/lib/Drupal/Core/File/FileUrlGenerator.php).

This occurs because the FileUrlGenerator expects a supported stream wrapper (public://, private://, etc.) but does not recognize or handle external URL schemes like ftp://.

Steps to reproduce

1. Create a field of type file_url.
2. Add content with an external URL value, such as ftp://example.com/file.txt.
3. Attempt to view the content.
4. The error mentioned above is displayed.

Proposed resolution

Update the RemoteFile class to handle external URL schemes by implementing a createFileUrl() method. This method will return the file’s URI directly using getFileUri(), bypassing the unsupported stream wrapper logic.