Part.write(String) is system dependent

The API [`Part.write(String)`](https://jakarta.ee/specifications/servlet/4.0/apidocs/javax/servlet/http/Part.html#write-java.lang.String-) takes a system dependent string that can either be a path or a filename. This can make it difficult to write portable code.

A possible solution is to provide alternative methods should be provided, one that takes a `File` and another that explicitly takes a URI path.

Start with the Part.write(String) API definition linked in the issue and review how its path-or-filename behavior is specified. Compare the proposed File and URI-path alternatives with the existing API conventions and compatibility requirements. Done means the API change and its portability semantics are agreed and specified; no test or implementation file is named.

Written by the indexing model from the issue text.