getFileName/getFileExtension not properly (re-) implemented for 1.x assets in AssetWrapper

info.magnolia.dam.asset.AssetWrapper#getFileExtension currently returns "" (or something if there happens to be a dot in the filename), because pre-2.0 assets had two properties: "fileName" and "extension".

The specs of are clear:

    /**
     * Returns the "preferred" file name for this asset's binary. The returned name should include the file's extension
     * (if any), but not its path.
     */
    String getFileName();

However, this doesn't make any assumptions/implications on the storage. One simple assumption to make in JcrAsset would be that "if property 'extension' exists, we're dealing with a 1.x Asset" and have getFileName() take that into account.
Ideally, UI components will lazily update such nodes (when edited), so that we don't actually have to migrate existing content.