Guess filetype for files without extensions

When evaluating file specifications to create file collections, we should follow this:

• If a file has a known extensions, mark it as text or binary based on the dictionary (implemented)
• Include files that do not have a file extension, and files with extensions not covered by the dictionary
  • Guess if the file is (canonically) text, otherwise mark them as binary
    • I'd prefer zlib's algorithm: https://github.com/madler/zlib/blob/master/doc/txtvsbin.txt
    • If a file does not have any content, then mark it as binary
• Document this flow in the specification section