title: Files.probeContentType() returns null on OpenJDK 11
body: <p>It seems that <code>Files.probeContentType()</code> returns <code>null</code> for a file (where the MIME type is not indicated by its file name) on OpenJDK 11, while it was working on OpenJDK 8 (on Linux x64). What is the reason for that?</p>
  <p>Note: It <em>may</em> work for files where the type is indicated by its name, e.g., &quot;test.pdf&quot; will produce &quot;application/pdf&quot;, but renaming the file to &quot;test&quot; will produce <code>null</code>.</p>