Update FoiAttachment factory

Simplify the after build callback by ensuring there will be a `body`,
`content_type` and `filename`.

spec/factories/foi_attchments.rb

@@ -4,17 +4,16 @@
     sequence(:url_part_number) { |n| n + 1 }
     display_size { '0K' }
     masked_at { 1.day.ago }
+    content_type { 'text/plain' }
+    filename { 'attachment.txt' }
 
     transient do
       body { 'hereisthemaskedtext' }
     end
 
     after(:build) do |foi_attachment, evaluator|
       body = evaluator.body
-      foi_attachment.hexdigest = Digest::MD5.hexdigest(body) if body
-
-      next unless body && foi_attachment.filename && foi_attachment.content_type
-
+      foi_attachment.hexdigest = Digest::MD5.hexdigest(body)
       foi_attachment.file.attach(
         io: StringIO.new(body),
         filename: foi_attachment.filename,

spec/models/foi_attachment_spec.rb

@@ -233,10 +233,12 @@
 
       context 'when attachment file is unattached' do
         let(:foi_attachment) do
-          FactoryBot.create(:body_text, filename: nil)
+          FactoryBot.create(:body_text)
         end
 
         it 'raises missing attachment exception' do
+          foi_attachment.file.purge
+
           expect { unmasked_body }.to raise_error(
             FoiAttachment::MissingAttachment,
             "file not attached (ID=#{foi_attachment.id})"