Simple mime type detection

 Marcel attempts to choose the most appropriate content type for a given file
 by looking at the binary data, the filename, and any declared type (perhaps
 passed as a request header).
 .
 By preference, the magic number data in any passed in file is used to determine
 the type. If this doesn't work, it uses the type gleaned from the filename,
 extension, and finally the declared type. If no valid type is found in any of
 these, "application/octet-stream" is returned.
 .
 Some types aren't easily recognised solely by magic number data. For example
 Adobe Illustrator files have the same magic number as PDFs (and can usually
 even be viewed in PDF viewers!). For these types, Marcel uses both the magic
 number data and the file name to work out the type.