Fix Generate/Regenerate/Scan buttons for newly uploaded media.

[iamdharmesh]

Description of the Change

As described in #304, generate/regenerate/rescan buttons are not working for newly uploaded images, This PR contains changes to make it work.

Closes #304

How to test the Change

1. Create a new post.
2. Add Image block and click "Media Library"
3. Switch to the "Upload files" tab in the media library.
4. Upload the image and wait for success and buttons in the attachment details sidebar.
5. Click on any Generate/Regenerate/Scan button and make sure it works fine.

Changelog Entry

Fixed - Generate/Regenerate/Scan buttons for newly uploaded media.

Credits

Props @iamdharmesh @jeffpaul @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.