[csharp-netcore] allow to specify content-type for files

[llRandom]

The changes allow to specify content-type for files sent using csharp-netcore client. If content-type is not set explicitly it will be set to application/octet-stream as it is currently set fo all files.

To close #10472

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[llRandom]

I've updated the author. Is it fine now?

[wing328]

Looks good now. Windows tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/40911871