liquibase-core/src/main/java/liquibase/util/file/FilenameUtils.java
public static String getExtension(String filename) {
        if (filename == null) {
            return null;
        }
        int index = indexOfExtension(filename);
        if (index == -1) {
            return "";
        } else {
            return filename.substring(index + 1);
        }
    }
public static String getExtension(String filename) {
        if (filename == null) {
            return null;
        }
        int index = indexOfExtension(filename);
        if (index == -1) {
            return "";
        } else {
            return filename.substring(index + 1);
        }
    }
Gets the extension of a filename.
<p>
This method returns the textual part of the filename after the last dot.
There must be no directory separator after the dot.
<pre>
foo.txt      --> "txt"
a/b/c.jpg    --> "jpg"
a/b.txt/c    --> ""
a/b/c        --> ""
</pre>
<p>
The output will be the same irrespective of the machine that the code is running on.

@param filename the filename to retrieve the extension of.
@return the extension of the file or an empty string if none exists.