Investigate if `BrowserFile` should be used instead bUnit's own `BunitBrowserFile`

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Follow up to #1518

Check if we can instantiate the "real" BrowserFile instead of our own BunitBrowserFile.

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@egil Took some time to investigate whether or not to pursue BrowserFile in contrast to our version (aka `BunitBrowserFile):

1. As BrowserFile is internal, creating the instance is only possible via Reflection. (Even not via UnsafeAccessorAttribute as the type has to be public).
2. Even if - the real Browserfile doesn't directly accept any content. Instead it wants the handle to the InputFile.
3. InputFile offers an internal OpenReadStream which maps to another BrowserFileStream which is also Internal.

Point 2 and 3 makes it way harder to offer the current API:

public static void UploadFiles(
	this IRenderedComponent<InputFile> inputFileComponent,
	params InputFileContent[] files)

We give the user a super easy way of faking the upload which works in many cases just fine (just for the image request it does not).

The user does not have to use UploadFiles. It is just an easy way, but as you highlighted to some extend in #415 the user could do it on his own.

I do believe we should document that specific exception and maybe find a way (with example code) to make it work without bUnit's BunitBrowserFile and UploadFiles. Furthermore, I am still convinced that the current approach has the better trade-offs overall!

[egil]

Ok sounds reasonable to me then. Let's go with what you suggest then.

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Closed by #1525