Bug 126705 [Preferences] FileFieldEditor does not call doCheckState
org.eclipse.jface.preference.FileFieldEditor does not call doCheckSate() during checkState() which makes it difficult to subclass this editor to perform additional checks like it is possible with StringFieldEditor
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/FileFieldEditor.java
* do not.
     */
    private File getFile(File startingDirectory) {
        FileDialog dialog = new FileDialog(getShell(), SWT.OPEN | SWT.SHEET);
        if (startingDirectory != null) {
			dialog.setFileName(startingDirectory.getPath());
		}
        if (extensions != null) {
			dialog.setFilterExtensions(extensions);
		}
        String file = dialog.open();
        if (file != null) {
            file = file.trim();
            if (file.length() > 0) {
				return new File(file);
			}
        }
        return null;
    }
    /**
     * Sets this file field editor's file extension filter.
     *
     * @param extensions a list of file extension, or <code>null</code> 
     * to set the filter to the system's default value
     */
    public void setFileExtensions(String[] extensions) {
        this.extensions = extensions;
    }
}