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
setErrorMessage(JFaceResources
                .getString("FileFieldEditor.errorMessage"));
        setChangeButtonText(JFaceResources.getString("openBrowse"));
        setValidateStrategy(validationStrategy);
        createControl(parent);
    }
    /* (non-Javadoc)
     * Method declared on StringButtonFieldEditor.
     * Opens the file chooser dialog and returns the selected file.
     */
    protected String changePressed() {
        File f = new File(getTextControl().getText());
        if (!f.exists()) {
			f = null;
		}
        File d = getFile(f);
        if (d == null) {
			return null;
		}
        return d.getAbsolutePath();
    }
    /* (non-Javadoc)
     * Method declared on StringFieldEditor.
     * Checks whether the text input field specifies an existing file.
     */
    protected boolean checkState() {
        String msg = null;
        String path = getTextControl().getText();
        if (path != null) {
			path = path.trim();