Admin User should be able to set Attachment file types that Users can upload.

[pdt256]

This is handled in the FileManagerInterface. The LocalFileManager implementation handles the allowed file types in the constructor:

public function __construct(
        $destinationPath,
        $uriPrefix = null,
        $allowedImageTypes = [
            IMAGETYPE_JPEG,
            IMAGETYPE_PNG
        ]
    )