Update attachment masking

We need to persist the attachment blob as the checksum and byte size can
change after masking.

app/models/foi_attachment.rb

@@ -96,6 +96,8 @@ def body=(d)
     ensure_filename!
     if file.attached?
       file_blob.upload(StringIO.new(d.to_s), identify: false)
+      file_blob.save
+
     else
       file.attach(
         io: StringIO.new(d.to_s),

spec/models/foi_attachment_spec.rb

@@ -125,6 +125,16 @@
       expect { attachment.update(body: 'masked', masked_at: Time.now) }.
         to_not change { attachment.file_blob.metadata }
     end
+
+    it 'persists changes to existing blob checksum' do
+      attachment = FactoryBot.create(
+        :foi_attachment, :unmasked, body: 'unmasked'
+      )
+
+      expect { attachment.update(body: 'masked', masked_at: Time.now) }.
+        to change { attachment.file_blob.checksum }
+      expect(attachment.file_blob.changed?).to eq false
+    end
   end
 
   describe '#body' do