Move attachable methods from helpers to models

We were using helper methods inside the model; we might as well include
them in the model and use them from anywhere else.

Note we're using a different logic for images and documents methods.
That's because for images the logic was defined in the helper methods,
but for documents the logic is defined in the Documentable concern. In
the past, different documentable classes allowed different content
types, while imageable classes have always allowed the same content
types.

I'm not sure which method is better; for now, I'm leaving it the way it
was (except for the fact that we're removing the helper methods).

app/models/document.rb

@@ -1,5 +1,4 @@
 class Document < ApplicationRecord
-  include DocumentablesHelper
   has_attached_file :attachment, url: "/system/:class/:prefix/:style/:hash.:extension",
                                  hash_data: ":class/:style/:custom_hash_data",
                                  use_timestamp: false,
@@ -48,6 +47,10 @@ class Document < ApplicationRecord
     attachment.instance.custom_hash_data(attachment)
   end
 
+  def self.humanized_accepted_content_types
+    Setting.accepted_content_types_for("documents").join(", ")
+  end
+
   def prefix(attachment, _style)
     if attachment.instance.persisted?
       ":attachment/:id_partition"
@@ -69,6 +72,14 @@ class Document < ApplicationRecord
     attachment_content_type.split("/").last.upcase
   end
 
+  def max_file_size
+    documentable_class.max_file_size
+  end
+
+  def accepted_content_types
+    documentable_class.accepted_content_types
+  end
+
   private
 
     def documentable_class
@@ -77,20 +88,18 @@ class Document < ApplicationRecord
 
     def validate_attachment_size
       if documentable_class.present? &&
-          attachment_file_size > documentable_class.max_file_size.megabytes
+          attachment_file_size > max_file_size.megabytes
         errors.add(:attachment, I18n.t("documents.errors.messages.in_between",
                                       min: "0 Bytes",
-                                      max: "#{documentable_class.max_file_size} MB"))
+                                      max: "#{max_file_size} MB"))
       end
     end
 
     def validate_attachment_content_type
-      if documentable_class &&
-         !accepted_content_types(documentable_class).include?(attachment_content_type)
-        accepted_content_types = documentable_humanized_accepted_content_types(documentable_class)
+      if documentable_class && !accepted_content_types.include?(attachment_content_type)
         message = I18n.t("documents.errors.messages.wrong_content_type",
                          content_type: attachment_content_type,
-                         accepted_content_types: accepted_content_types)
+                         accepted_content_types: self.class.humanized_accepted_content_types)
         errors.add(:attachment, message)
       end
     end