Blocks have had a major overhaul in Drupal 8.
Blocks are now broken down into two modules. The Block module simply handles the placement of blocks into regions. The Custom Block module implements a new entity that allows you create block types and add fields to them.
In these two videos, taken from our upcoming Drupal 8 Site Building class, Rod introduces you to blocks in Drupal 8.
In this first video, you'll see how to create a new block type, with a file field:
In this second video, you'll see how to use block types, with a file field, and manage them via your custom block library.