fix submission result

lib/submissions/generate_result_attachments.rb

@@ -49,7 +49,7 @@ module Submissions
                 attachment.download
               else
                 Vips::Image.new_from_buffer(attachment.download, '')
-                           .write_to_buffer('.jpg', Q: 40)
+                           .write_to_buffer('.png')
               end
 
             io = StringIO.new(image_data)

lib/templates/process_document.rb

@@ -15,7 +15,7 @@ module Templates
     def call(attachment)
       if attachment.content_type == PDF_CONTENT_TYPE
         generate_pdf_preview_images(attachment)
-      elsif attachment.content_type.starts_with?('image/')
+      elsif attachment.image?
         generate_preview_image(attachment)
       end