为每个文件确定Files.probeFileContent(Path)的文件的内容类型时，结果为null。
环境：
java version "1.8.0"
Java(TM) SE Runtime Environment (build pap6480sr1fp1-20150603_01(SR1 FP1))
IBM J9 VM (build 2.8, JRE 1.8.0 AIX ppc64-64 Compressed References 20150410_243669 (JIT enabled, AOT enabled)
J9VM - R28_Java8_SR1_20150410_1531_B243669
JIT - tr.r14.java_20150402_88976.03
GC - R28_Java8_SR1_20150410_1531_B243669_CMPRSS
J9CL - 20150410_243669)
JCL - 20150601_01 based on Oracle jdk8u45-b13