Select Multiple Attachments

I have a question: is it possible to create an attachment field that selects multiple pictures / files?

There isn’t, but there’s a pull request in-progress for it: https://github.com/wp-shortcake/shortcake/pull/508

If not, is there any method to create a custom field type to use with the UI?

There is an API for registering your own custom fields, but doing so would require a substantial amount of development effort.