Make S3 serve attachment using the content disposition header

app/models/attachment.rb

@@ -60,7 +60,7 @@ class Attachment < ActiveRecord::Base
   # Returns an URL if the attachment is stored in an external (fog) attachment storage
   # or nil otherwise.
   def external_url
-    url = URI.parse file.download_url # returns a path if local
+    url = URI.parse file.download_url(content_disposition: content_disposition) # returns a path if local
 
     url if url.host
   rescue URI::InvalidURIError

app/uploaders/file_uploader.rb

@@ -49,7 +49,7 @@ module FileUploader
     file.to_file
   end
 
-  def download_url
+  def download_url(options = {})
     file.is_path? ? file.path : file.url
   end
 

app/uploaders/fog_file_uploader.rb

@@ -57,8 +57,20 @@ class FogFileUploader < CarrierWave::Uploader::Base
     super
   end
 
-  def download_url
-    remote_file.url
+  def download_url(options = {})
+    url_options = {}
+
+    if options[:content_disposition].present?
+      url_options[:query] = {
+        # Passing this option to S3 will make it serve the file with the
+        # respective content disposition. Without it no content disposition
+        # header is sent. This only works for S3 but we don't support
+        # anything else anyway (see carrierwave.rb).
+        "response-content-disposition" => options[:content_disposition]
+      }
+    end
+
+    remote_file.url url_options
   end
 
   ##

spec/models/attachment_spec.rb

@@ -233,4 +233,59 @@ describe Attachment, type: :model do
       expect(File.exists?(attachment.file.path)).to eq false
     end
   end
+
+  describe "#external_url" do
+    let(:author) { FactoryBot.create :user }
+
+    let(:image_path) { Rails.root.join("spec/fixtures/files/image.png") }
+    let(:text_path) { Rails.root.join("spec/fixtures/files/testfile.txt") }
+    let(:binary_path) { Rails.root.join("spec/fixtures/files/textfile.txt.gz") }
+
+    let(:image_attachment) { Attachment.new author: author, file: File.open(image_path) }
+    let(:text_attachment) { Attachment.new author: author, file: File.open(text_path) }
+    let(:binary_attachment) { Attachment.new author: author, file: File.open(binary_path) }
+
+    before do
+      Fog.mock!
+
+      connection = Fog::Storage.new provider: "AWS"
+      connection.directories.create key: "my-bucket"
+
+      CarrierWave::Configuration.configure_fog! credentials: {}, directory: "my-bucket", public: false
+
+      # Remounting the uploader overrides the original file setter taking care of setting,
+      # among other things, the content type. So we have to restore that original
+      # method this way.
+      Attachment.alias_method :set_file, :file=
+      Attachment.mount_uploader :file, FogFileUploader
+      Attachment.alias_method :file=, :set_file
+    end
+
+    describe "for an image file" do
+      before { image_attachment.save! }
+
+      it "should make S3 use content_disposition inline" do
+        expect(image_attachment.content_disposition).to eq "inline"
+        expect(image_attachment.external_url.to_s).to include "response-content-disposition=inline"
+      end
+    end
+
+    describe "for a text file" do
+      before { text_attachment.save! }
+
+      it "should make S3 use content_disposition inline" do
+        expect(text_attachment.content_disposition).to eq "inline"
+        expect(text_attachment.external_url.to_s).to include "response-content-disposition=inline"
+      end
+    end
+
+    describe "for a binary file" do
+      before { binary_attachment.save! }
+
+      it "should make S3 use content_disposition 'attachment; filename=...'" do
+        expect(binary_attachment.content_disposition).to eq "attachment; filename=textfile.txt.gz"
+        expect(binary_attachment.external_url.to_s).to include "response-content-disposition=attachment%3B%20filename%3Dtextfile.txt.gz"
+      end
+    end
+  end
 end