Use file_validators to validate attachments

We were using custom rules because of some issues with Paperclip. These
rules work fine, but since we're already using the file_validators gem,
we might as well simplify the code a little bit.

spec/shared/models/image_validations.rb

@@ -38,7 +38,7 @@ shared_examples "image validations" do |imageable_factory|
 
   it "is not valid for attachments larger than imageable max_file_size definition" do
     larger_size = Setting["uploads.images.max_size"].to_i.megabytes + 1.byte
-    allow(image).to receive(:attachment_file_size).and_return(larger_size)
+    allow(image.attachment).to receive(:byte_size).and_return(larger_size)
 
     expect(image).not_to be_valid
     expect(image.errors[:attachment]).to include "must be in between 0 Bytes and 1 MB"