RM4069 - Append the file suffix to filenames when needed in the FileCreate dialogue.

I'd normally commit something like this directly but wanted some thoughts on the fact that it introduces a third method for figuring out what path separator to use to the file manager code:

1) We get it from the config file (which is actually a template)

2) We look at what's already in an existing path

and now:

3) We use navigator.platform 

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