Prevent new line insertion while attaching a file

What does this MR do and why?

Fixes a regression caused by !92680 (merged), where clicking Attach a file or image button causes a new line insertion in the markdown field.

Screenshots or screen recordings

NA

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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)