Fix import in Windows 10 + Chrome

[dasanra]

What it solves

Resolves #2578

How this PR fixes it

It allows file type not to be defined but also includes a file extension validation in order to still do a minimum check that a valid file is being uploaded

How to test it

It can only be tested using Chrome in Windows 10
If an office suite is installed it may be possible that the issue is not repeatable anymore. I can just confirm that it's possible with a clean Windows 10 or with LibreOffice installed

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ESLint Summary View Full Report

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	1	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

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[tschubotz]

I just tested this in my Win10 VM and I was able to successfully import the csv from the issue!