Preview Active Storage uploads after attach using Turbo Streams

For image uploads, I like immediate previews. With Active Storage, you can render the preview server-side once the blob is attached, and use Turbo Streams to update the preview area. The form submits to an endpoint that attaches the blob and returns a stream replacing #image_preview with a partial that renders an image_tag variant. This keeps image processing logic centralized and avoids client-side FileReader code. It’s also consistent: the preview looks like the final show page. The key is handling validation and returning 422 when the attach fails, updating an error target. If you want truly instant previews before upload, you can add Stimulus, but server-side previews are often enough and much simpler.