Make BrowserFile testable

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

In its current state `BrowserFile` is an `internal sealed class` which makes it impossible to unit-test from the outside world.

Especially if you have functions like [`BrowserFileExtensions.RequestImageFileAsync`](https://github.com/dotnet/aspnetcore/blob/main/src/Components/Web/src/Forms/InputFile/BrowserFileExtensions.cs#L29) which takes an `IBrowserFile` but throws exceptions when invoked with everything besides the concrete **internal** `BrowserFile` type.

### Describe the solution you'd like

The easiest and least invasive solution would be to make the `BrowserFile` type `public` and not `sealed`.

### Additional context

_No response_