WordPress function to add text

As your hook only passes the $post_content and $attachment_id, you’ll first need to get the relevant post, and then you can output your required data.

Please try this and comment if it does not meet your requirements.

    function myprefix_change_afip_post_content( $post_content, $attachment_id ) {

            global $post
            $post = get_post( $attachment_id );
            setup_postdata( $post );

            $my_uploaded_image = wp_get_attachment_image_src( $attachment_id );

            $post_content = sprintf(
                    'This post – %1$s is uploaded by %2$s on %3$s on category %4$s',
                    get_the_title(),
                    get_the_author_meta( 'display_name' );
                    get_the_date(),
                    get_the_category()
            );

            wp_reset_postdata()

            return $post_content;

    }

Note – this will overwrite the previous $post_content. If you want to add to it instead simply replace $post_content = sprintf( with $post_content.= sprintf(.