Enable autogeneration of preview/thumbnail

Whenever uploading a high res file, automatically create a low res preview/thumbnail so that:

1. we do not necessarily need to add a downloadable for the high res version
2. the preview would not be of much interest if it were "stolen"

I still see a lot of high res files uploaded as "preview". It not only takes time to display, but also poses a risk of anyone being able to copy the high res file without purchasing the item.