FileUpload follow ups

[pranavkm]

• Harden the documentation for IBrowserFile to make it clear that these properties are provided by the client and are untrusted
• Do some prelimnary validation on IBrowserFile. For e.g. throw if File.Size is negative.
• Consider renaming ToImageFileAsync -> RequestImageFileAsync
• Add a InputFile.MaxFileSize = 15MB and a InputFile.MaxAllowedFiles = 3
• Don't document samples with MemoryStream

[mkArtakMSFT]

This is done as part of #25248