support selective file exports with select[files] param

this is a friendly alias for select[attachments], done to be
more consistent with existing API names, and following the
example of select[modules], select[pages], etc.

test plan:
 - do selective content exports with the content export API,
   using select[files][]=(id), and ensure the correct file(s)
   are exported
 - test both common cartridge and zip exports

fixes CNVS-15129

Change-Id: I004ae1e8ba28ea8b7cf480c8fee5bd9633375781
Reviewed-on: https://gerrit.instructure.com/40115
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/models/content_export.rb

@@ -217,6 +217,14 @@ class ContentExport < ActiveRecord::Base
     self.settings[:selected_content] ||= {}
   end
 
+  def select_content_key(obj)
+    if zip_export?
+      obj.asset_string
+    else
+      CC::CCHelper.create_key(obj)
+    end
+  end
+
   # Method Summary
   #   Takes in an ActiveRecord object. Determines if the item being 
   #   checked should be exported or not. 
@@ -231,7 +239,7 @@ class ContentExport < ActiveRecord::Base
     return true if is_set?(selected_content["all_#{asset_type}"])
 
     return false unless selected_content[asset_type]
-    return true if is_set?(selected_content[asset_type][CC::CCHelper.create_key(obj)])
+    return true if is_set?(selected_content[asset_type][select_content_key(obj)])
 
     false
   end
@@ -253,7 +261,7 @@ class ContentExport < ActiveRecord::Base
 
     asset_type = type || obj.class.table_name
     selected_content[asset_type] ||= {}
-    selected_content[asset_type][CC::CCHelper.create_key(obj)] = true
+    selected_content[asset_type][select_content_key(obj)] = true
   end
   
   def add_error(user_message, exception_or_info=nil)

app/models/content_migration.rb

@@ -626,6 +626,8 @@ class ContentMigration < ActiveRecord::Base
       'content_tags'
     when 'pages'
       'wiki_pages'
+    when 'files'
+      'attachments'
     else
       key
     end

spec/apis/v1/content_exports_api_spec.rb

@@ -359,6 +359,13 @@ describe ContentExportsApiController, type: :request do
                                  { :select => {:pages => [page_to_copy.id]} })
         export3 = t_course.content_exports.find_by_id(json3['id'])
         export3.export_object?(page_to_copy).should be_true
+
+        file = attachment_model(context: t_course)
+        json4 = api_call_as_user(t_teacher, :post, "/api/v1/courses/#{t_course.id}/content_exports?export_type=common_cartridge",
+                                 { controller: 'content_exports_api', action: 'create', format: 'json', course_id: t_course.to_param, export_type: 'common_cartridge'},
+                                 { :select => {:files => [file.id]} })
+        export4 = t_course.content_exports.find_by_id(json4['id'])
+        export4.export_object?(file).should be_true
       end
 
       it "should export by module item id" do
@@ -465,6 +472,14 @@ describe ContentExportsApiController, type: :request do
         end
       end
 
+      it "should support 'files' in addition to 'attachments'" do
+        json = api_call_as_user(t_teacher, :post, "/api/v1/courses/#{t_course.id}/content_exports",
+                                { controller: 'content_exports_api', action: 'create', format: 'json', course_id: t_course.to_param },
+                                { export_type: 'zip', select: {'files' => [@file1.id]} })
+        ce = ContentExport.find(json['id'])
+        ce.export_object?(@file1).should be true
+      end
+
       context "as a student" do
         it "should exclude non-zip and/or other users' exports from #index" do
           my_zip_export = past_export(t_course, t_student, 'zip')