Add document model validations and model shared specs.

app/models/document.rb

@@ -1,8 +1,14 @@
 class Document < ActiveRecord::Base
+  has_attached_file :attachment
+
   belongs_to :user
   belongs_to :documentable, polymorphic: true
 
-  validates :user_id, presence: true
+  validates_attachment :attachment, presence: true,
+    content_type: { content_type: "application/pdf" },
+    size: { in: 0..3.megabytes }
+  validates :title, presence: true
+  validates :user, presence: true
   validates :documentable_id, presence: true
   validates :documentable_type, presence: true