why does Files.probeContentType return null

I have reported the bug to oracle again, hoping they will backport the jdk8 solution (I don't have much hope but you never know).

In the meantime you can use my own backport of the FileTypeDetector available at https://github.com/jeantil/jdk7-mimeutils the maven project packages to a jar which can be added to your classpath to enable mime type detection. I also provide a mime.types file to put in your home folder for the detection to work correctly. I extracted the mime.types file from some version of apache so it's pretty complete.

I found that the FileTypeDetector is buggy on OS X: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7133484

Apparently this will be fixed in Java 8.