attachment-picker Stimulus controller does not sync hidden fields after inline upload #1425

The problem
When using Spina::Parts::Attachment in the admin page editor, uploading a new file via the inline upload button results in signed_blob_id and filename being saved as empty strings, even though the Spina::Attachment record is created correctly and the file is attached.

This causes Spina::Parts::Attachment#present? to return false (since it checks signed_blob_id.present?), and ContentPresenter#attachment_url to return nil, making the uploaded file inaccessible in frontend views.

Root Cause

There are two issues in the interaction between the attachment-picker Stimulus controller and the inline upload Turbo Stream response:

1. pick() is never called after inline upload

The attachment-picker controller's pick() method is bound to the change event on the <select> element. When the inline upload Turbo Stream response appends a new <option selected> to the <select>, the browser does not fire a change event (programmatic DOM mutations don't trigger input events). Therefore pick() is never invoked, and the hidden fields signed_blob_id and filename remain at their previous values (empty strings for a new part).

2. Attribute format mismatch between dropdown options and inline upload options

Even if pick() were called, there is an attribute format mismatch:

• Dropdown options (rendered by the Rails select helper in _form.html.erb) use data-* attributes:

  <option value="1" data-signed-blob-id="eyJ..." data-filename="video.mp4">video.mp4</option>

• Inline upload options (rendered by parts/attachments/_attachment.html.erb) use plain HTML attributes:

  <option value="1" signed-blob-id="eyJ..." filename="video.mp4" selected>video.mp4</option>

The pick() method reads from option.dataset.signedBlobId and option.dataset.filename, which correspond to data-signed-blob-id and data-filename attributes respectively. These are undefined on inline upload options, so even a manual re-selection after upload would write empty strings.

Steps to Reproduce

1. Create a page with a part that uses Spina::Parts::Attachment
2. Edit the page in Spina admin
3. In the Attachment field, click "Upload" and select a file
4. Save the page
5. Inspect the page's JSON content — signed_blob_id will be "" and filename will be ""
6. page.content(:your_attachment_part).present? returns false

Workaround: After the inline upload completes, manually re-select the same file from the dropdown. This triggers the change event, pick() reads the data-* attributes from the dropdown option (which has them correctly), and the hidden fields are populated.

Environment

• Ruby 3.4.2
• Spina 2.20.0
• Rails 8.0.2
• attr_json 2.5.1
• Stimulus 3.x (via @hotwired/stimulus)
• Turbo (via @hotwired/turbo-rails)