Update UX for attaching files in GFM input fields

What does this MR do and why?

Updates Attach a file button placement to be an icon button in GFM field toolbar, while showing only upload progress in the status bar.

Screenshots or screen recordings

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How to set up and validate locally

1. Checkout 363285-update-file-attach-button-gfm branch locally.
2. Open any page where there's a GFM input field present.
3. Look for 📎 button in the GFM toolbar and click on it to upload files.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363285 (closed)