attestation: fix preview of logos

Turns out that assigning the params to the procedure calls `read` on the
images files. Calling `read` moves the seek offset to the end of the
file; which means that subsequent calls to `read` return an empty
string.

Fix the issue by calling `rewind` before attempting to `read` the file,
to ensure the seek offset is at the beginning of the file.

Fix #4958

app/views/admin/attestation_templates/show.pdf.prawn

@@ -38,12 +38,12 @@ prawn_document(margin: [top_margin, right_margin, bottom_margin, left_margin], p
 
   pdf.bounding_box([0, pdf.cursor], width: body_width, height: body_height) do
     if logo.present?
-      logo_file = if logo.is_a?(ActiveStorage::Attached::One)
+      logo_content = if logo.is_a?(ActiveStorage::Attached::One)
         logo.download
       else
-        logo.read
+        logo.rewind && logo.read
       end
-      pdf.image StringIO.new(logo_file), fit: [max_logo_width , max_logo_height], position: :center
+      pdf.image StringIO.new(logo_content), fit: [max_logo_width , max_logo_height], position: :center
     end
 
     pdf.fill_color grey
@@ -57,12 +57,12 @@ prawn_document(margin: [top_margin, right_margin, bottom_margin, left_margin], p
 
     if signature.present?
       pdf.pad_top(40) do
-        signature_file = if signature.is_a?(ActiveStorage::Attached::One)
+        signature_content = if signature.is_a?(ActiveStorage::Attached::One)
           signature.download
         else
-          signature.read
+          signature.rewind && signature.read
         end
-        pdf.image StringIO.new(signature_file), fit: [max_signature_size , max_signature_size], position: :right
+        pdf.image StringIO.new(signature_content), fit: [max_signature_size , max_signature_size], position: :right
       end
     end
   end

spec/controllers/admin/attestation_templates_controller_spec.rb

@@ -46,6 +46,14 @@ describe Admin::AttestationTemplatesController, type: :controller do
     context 'if an attestation template does not exist on the procedure' do
       let(:attestation_template) { nil }
 
+      context 'with images' do
+        let(:upload_params) { { title: 't', body: 'b', footer: 'f', logo: interlaced_logo } }
+
+        it { expect(assigns(:attestation)).to include({ title: 't', body: 'b', footer: 'f' }) }
+        it { expect(assigns(:attestation)[:logo]).to be_present }
+        it_behaves_like 'rendering a PDF successfully'
+      end
+
       context 'without images' do
         let(:upload_params) { { title: 't', body: 'b', footer: 'f' } }