RFR of 8129632: (fs) Files.probeContentType returns null on Mac OS X

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8129632
Patch:	http://cr.openjdk.java.net/~bpb/8129632/webrev.00/

Summary:

1) Added OS X-specific UTIFileTypeDetector using Uniform Type Identifiers to map file extensions to content types.
2) Modified MacOSXFileSystemProvider to return a chain of MimeTypesFileTypeDetector followed by UTIFileTypeDetector.
3) Moved getExtension() up from MimeTypesFileTypeDetector to parent class AbstractFileTypeDetector.
4) Extended content type detection in Basic test to some other common file extensions.

The jdk_nio regression tests including the one modified by this patch passed on all the usual platforms.

Thanks,

Brian