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
} else {
			path = "";
		}
        if (path.length() == 0) {
            if (!isEmptyStringAllowed()) {
				msg = getErrorMessage();
			}
        } else {
            File file = new File(path);
            if (file.isFile()) {
                if (enforceAbsolute && !file.isAbsolute()) {
					msg = JFaceResources
                            .getString("FileFieldEditor.errorMessage2");
				}
            } else {
                msg = getErrorMessage();
            }
        }
        if (msg != null) { 
            showErrorMessage(msg);
            return false;
        }
        
        clearErrorMessage();
        return true;
    }
    /**
     * Helper to open the file chooser dialog.
     * @param startingDirectory the directory to open the dialog on.
     * @return File The File the user selected or <code>null</code> if they