translate user content links to replaced files

changes the user content html rewriter so links
to files that were deleted and replaced will still
be translated correctly (rather than pointing to
the old deleted file)

test plan:
* upload a file
* create an assignment
* add a link to the file in the assignment description
* delete the file
* edit the file locally and re-upload it
* edit the assignment and save it
* should still be able to click on the link
 and receive the updated file

fixes #CNVS-4799

Change-Id: I527756f02d7b38dda36dbcc3cfae32a3980a25b3
Reviewed-on: https://gerrit.instructure.com/18992
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

lib/api.rb

@@ -267,7 +267,13 @@ module Api
 
     rewriter = UserContent::HtmlRewriter.new(context, user)
     rewriter.set_handler('files') do |match|
-      obj = match.obj_id && match.obj_class.find_by_id(match.obj_id)
+      if match.obj_id
+        if match.obj_class == Attachment && context && !context.is_a?(User)
+          obj = context.attachments.find(match.obj_id) rescue nil
+        else
+          obj = match.obj_class.find_by_id(match.obj_id)
+        end
+      end
       next unless obj && rewriter.user_can_view_content?(obj)
       file_download_url(obj.id, :verifier => obj.uuid, :download => '1', :host => host, :protocol => protocol)
     end

spec/apis/user_content_spec.rb

@@ -38,6 +38,29 @@ describe UserContent, :type => :integration do
     doc.at_css('img')['src'].should == "http://www.example.com/files/#{@attachment.id}/download?verifier=#{@attachment.uuid}"
   end
 
+  it "should translate file links to directly-downloadable urls for deleted and replaced files" do
+    course_with_teacher(:active_all => true)
+    attachment_model
+    @attachment.destroy
+    attachment2 = Attachment.create!(:folder => @attachment.folder, :context => @attachment.context, :filename => @attachment.filename, :uploaded_data => StringIO.new("first"))
+    @context.attachments.find(@attachment.id).id.should == attachment2.id
+
+    @assignment = @course.assignments.create!(:title => "first assignment", :description => <<-HTML)
+    <p>
+      Hello, students.<br>
+      This will explain everything: <img src="/courses/#{@course.id}/files/#{@attachment.id}/preview" alt="important">
+    </p>
+    HTML
+
+    json = api_call(:get,
+                    "/api/v1/courses/#{@course.id}/assignments/#{@assignment.id}",
+                    { :controller => 'assignments_api', :action => 'show',
+                      :format => 'json', :course_id => @course.id.to_s, :id => @assignment.id.to_s })
+
+    doc = Nokogiri::HTML::DocumentFragment.parse(json['description'])
+    doc.at_css('img')['src'].should == "http://www.example.com/files/#{attachment2.id}/download?verifier=#{attachment2.uuid}"
+  end
+
   it "should translate media comment links to embedded video tags" do
     course_with_teacher(:active_all => true)
     attachment_model