slide editor filename extension should default to .html

[Filipe Cruz]

For a long time i believed the slide editor was just broken or needing special server privileges, i was naively assuming it rendered a jpg or png from the html text inputted. Today i discovered it just saves the input as text and that one should name the file with the .html extension for it. Thought it might be nice little usability help for first time users to add .html by default (or clue it more clearly next to the filename box) to ensure it will work.