Option to add custom type mapping

[antal-huck]

I have a C# class with a property of type IFormFile and I would like to have CSharpToTypeScript transform this property to the type of a TypeScript File.

I suggest adding the option to add custom mapping rules (similar to Dates), e.g.

--addCustomMapping IFormFile File

[AdrianWilczynski]

Custom mapping option is a really good idea though in the case of IFormFile maybe we could just make File a default?

[antal-huck]

Yes, indeed.