don't require file extension for files to be copied

[mjkkirschner]

Purpose

change was made here to require file paths, I'm not sure why. #11798
@QilongTang can you take a look too see if removing this check is ok?

see an issue here #13617

I think the use case is definitely valid, many files don't have an extension.

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• The codebase is in a better state after this PR
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• User facing strings, if any, are extracted into *.resx files
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• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

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[QilongTang]

@mjkkirschner I agree, seems too much of a restriction

[QilongTang]

@mjkkirschner Do you think this would be good to include in 2.17.0, feel this one is not so risky