Validate alt text for attachments?

The code below will only run once whenever a file is uploaded.

1. We keep an array ( $image_mimes ) of acceptable image-mimetypes
2. We get the current attachment mime type
3. We make sure what is given is indeed an image ( because we don’t need unnecessary postmeta cluttering our table )
4. We grab the title from the attachment and set it to the alt-text initially

After that the user can update it however they wish ( or remove it entirely ):

function add_image_alt( $attachment_id ) {
    $image_mimes     = array( 'image/jpeg', 'image/gif', 'image/png', 'image/bmp', 'image/tiff', 'image/x-icon' );
    $attachment_type = get_post_mime_type( $attachment_id );

    if( in_array( $attachment_type, $image_mimes ) ) {
        $attachment_title = get_the_title( $attachment_id );
        update_post_meta( $attachment_id, '_wp_attachment_image_alt', sanitize_text_field( $attachment_title ) );
    }
}
add_action( 'add_attachment', 'add_image_alt' );