Don't get "IMAGINARY" files type

Imaginary files are files that does not exist, so it's useless to get
them, see http://commons.apache.org/proper/commons-vfs/apidocs/org/apache/commons/vfs2/FileType.html#IMAGINARY for more details on this file type.

In my case they are broken symbolic links, that must not be shown.

An other way to do would be to set these files as hidden, in this case we should
find an icon to associate. But as a such file doesn't exist i made this PR in this way.