fix: prevent non-file drags from showing upload overlay

[anirudhu-ui]

Summary

Fixes the page-level upload overlay so it only responds to real file drags. This prevents normal in-app drag interactions, like dragging the Brightness slider, from being treated as a video upload attempt.

Root cause

The document-level drag handlers in FileUpload.tsx were calling preventDefault() and showing the overlay for every drag event. That meant non-file drag events from UI controls could activate the global "Drop your video anywhere" overlay.

Changes

• Added a small isFileDrag guard based on dataTransfer.types.
• Applied the guard to the global dragenter, dragleave, dragover, and drop handlers.
• Kept external file drag-and-drop behavior intact.
• Added regression coverage for file and non-file document drag events.

Verification

• npm test -- --run src/components/__tests__/FileUpload.test.tsx
• npm test -- --run
• npm run lint
• npm run build

Fixes #1515

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