#548 check file contents for binary while uploading

[Lee Worden]

ImportProjectFiles uses files' extensions to decide whether to offer to import them to text or image pages. When you try to upload a file, it also checks the file contents on the server side and warns if you're trying to put a binary file on a text page. It would be good to also do this check on the client side and avoid giving you the wrong page suggestion, since there are many filename extensions that we aren't familiar with.