Enhancement: Toggle add file button on single file field

[heybran]

Description

On file field, if it's a single file field (file_limit equals to 1), once an file has been added, we can hide the Add file button, once this file is deleted, then the button will show again, so the original situation reported on the issue can be fixed.

Related GitHub issue(s)

#7120

Testing instructions

1. Create a single file field
2. Go to post type
3. First you will see the Add file button
4. Add a file then Add file button will be hidden
5. Once file is deleted, Add file button will be shown again

Screenshots / screencast

Screen.Recording.2024-06-21.at.12.40.11.mov

Changelog text for these changes

Enhancement: Single file field now have better user experience. (@heybran)

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).