yes to all this, but then when do you stop? how many file types should be supported by a browser? i'm sure some people would like it that the browser could edit spreadsheets too. there are so many different use cases.
Why not?
Why shouldn't browsers display at least the most common image, document and media types (including spreadsheets?) Why only display a few content-types but download all others? Content is the point of the web, after all.