include media_entry_id in media_preview endpoint

test plan:
* previews for media files in new files should work

closes #CNVS-18761

Change-Id: I5499f64d7a0f2a6a9efb66a7caae20df40ae2678
Reviewed-on: https://gerrit.instructure.com/49222
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/coffeescripts/bundles/file_preview.coffee

@@ -5,4 +5,4 @@ require [
 
   $preview = $("#media_preview")
   data = $preview.data()
-  $preview.mediaComment('show_inline', 'maybe', data.type, data.download_url)
+  $preview.mediaComment('show_inline', data.media_entry_id || 'maybe', data.type, data.download_url)

app/controllers/files_controller.rb

@@ -552,6 +552,8 @@ class FilesController < ApplicationController
           }
         }
 
+        json[:attachment][:media_entry_id] = attachment.media_entry_id if attachment.media_entry_id
+
         if (params[:verifier] && params[:verifier] == attachment.uuid) ||
             attachment.grants_right?(@current_user, session, :download)
           # Right now we assume if they ask for json data on the attachment

app/helpers/attachment_helper.rb

@@ -44,6 +44,7 @@ module AttachmentHelper
   def media_preview_attributes(attachment, attrs={})
     attrs[:type] = attachment.content_type.match(/video/) ? 'video' : 'audio'
     attrs[:download_url] = context_url(attachment.context, :context_file_download_url, attachment.id)
+    attrs[:media_entry_id] = attachment.media_entry_id if attachment.media_entry_id
     attrs.inject("") { |s,(attr,val)| s << "data-#{attr}=#{val} " }
   end