fix: Check drag event contains files before showing "copy" cursor

[tommoor]

What kind of change does this PR introduce?

• bugfix
• feature
• refactoring / style
• build / chore
• documentation

Did you add tests for your changes?

• Yes, my code is well tested
• Not relevant

If relevant, did you update the documentation?

• Yes, I've updated the documentation
• Not relevant

Summary

A green "plus" symbol shows when dragging links and other non-file items across a dropzone. This is particularly noticable if you combine react-dropzone with a draggable library like react-dnd, but it's also noticeable in the examples even in the docs.

closes #1042

[coveralls]

Pull Request Test Coverage Report for Build 63b7ff8ce2c291b23223782937ccca394ababec9-PR-1053

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 99.735%

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Totals
Change from base Build 6f0e826f6337367ab8b47efaa04449d839594c2a:	0.3%
Covered Lines:	205
Relevant Lines:	205

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 63b7ff8ce2c291b23223782937ccca394ababec9-PR-1053

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 99.735%

---

Totals
Change from base Build 6f0e826f6337367ab8b47efaa04449d839594c2a:	0.3%
Covered Lines:	205
Relevant Lines:	205

---

💛 - Coveralls

[github-actions]

🎉 This PR is included in version 11.3.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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