support for handling file uploads

This adds handling of file uploads by Base64-encoding the content and storing it in the session.
Done. I'm already working on extracting the code for saving & loading of step data into a separate class. I'm not sure if I'll keep the `handleFileUploads` and `handleFileUploadsTempDir` attributes in the `FormFlow` class or move them to the new class. But keeping them would allow setting them on a per-flow basis.