Fix guide images on production

# Why?

 * Guide images were broke only in production

# What Changed?

 * Removed no longer needed `Attachment` class
 * Stopped calling `featured_image=` async because it is no longer the way we do that.

# What's Next?

 *  `Stages::CreateStage#set_pictures` is the next suspect. Looks like it is setting attributes but not saving changes. Can't confirm yet, stay tuned.