Magic number and bug fixed

[J-Christophe]

• Bug fixed :
  I found a small problem in your code. It seems that no exception was send when a file was given as parameter of util.findMatch(...). Here is the problem in your code
  `

  
  
  if (file.length() < readSize) {
  
  readSize = (int) file.length();
  
  }
  
  if (readSize == 0) {
  
  return ContentInfo.EMPTY_INFO;
  
  }

` file.length does not send an Exception if the file does not exist but 0L. Then, when the file does not exist, ContentInfo.EMPTY_INFO is returned. I added a condition to check the file is well readable (if it is readable then it exists).

• Feature
  I added the following method
  public ContentInfo findMatch(final URL url) throws IOException

• Magic number
  I added the magic number for VOTABLE (http://www.ivoa.net/documents/VOTable/), XSD (I did not find a mime type), OGG (I do not know how to make the difference between sound and video)

• MimeType
  I added the mime type for netcdf (I was not written with the magic number expression)

[j256]

Sorry for the delay on this. If you can break these into individual PRs then I'll take them but I don't like to mix issues. I'm also happy to split them up myself. Thanks.