[PowerShell][Bug] Fix issue #18428 - [System.IO.FileInfo] object used in multipart/form-data submission does not support relative paths

[condorcorde]

When the path of an object of type System.IO.FileInfo is relative, the full path is calculated using as directory the value returned by [System.IO.Directory]::GetCurrentDirectory(), which is a constant rather than the value returned by Get-Location. See Issue #18428

This fix uses the method GetUnresolvedProviderPathFromPSPath to resolve a path relative to the current directory. Only form parameters of type System.IO.FileInfo are affected.

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  @wing328

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50317926

[wing328]

looks good. thanks for the PR

let's give it a try