Improve folder upload error message

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

#525474 (closed)

When users try to upload files by clicking the + button next to the branch name and selecting "Upload file," then attempt to drag and drop a folder, the system shows an error message that isn't user-friendly or clear enough.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the project repository page
2. Click the + button next to the branch name
3. Select "Upload file"
4. Try to drag and drop a folder into the upload area
5. Observe the error message

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.