show media preview for media files in file show view

test plan:
* upload a media file (video/audio) to a course
* when either:
 * linking the file to a module item and
 clicking on the item link
 * or, directly going to the file show page
 (e.g. '/courses/:course_id/files/:attachment_id')
* confirm that the media preview appears (rather
 than a failed document preview), just as on
 the main 'Files' page

fixes #CNVS-2529

Change-Id: Ic8701c28a7ea0382de84e36fc363c0cff8cc2ac6
Reviewed-on: https://gerrit.instructure.com/21616
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>

app/coffeescripts/bundles/file_preview.coffee

@@ -0,0 +1,8 @@
+require [
+  'jquery'
+  'media_comments'
+], ($) ->
+
+  $preview = $("#media_preview")
+  data = $preview.data()
+  $preview.mediaComment('show_inline', 'maybe', data.type, data.download_url)

app/helpers/attachment_helper.rb

@@ -45,4 +45,10 @@ module AttachmentHelper
     end
     attrs.inject("") { |s,(attr,val)| s << "data-#{attr}=#{val} " }
   end
+
+  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.inject("") { |s,(attr,val)| s << "data-#{attr}=#{val} " }
+  end
 end

app/views/files/show.html.erb

@@ -35,6 +35,13 @@
   <% elsif @attachment.content_type && @attachment.content_type.match(/\Aimage\//) %>
     <h2><%= @attachment.display_name %></h2>
     <%= link_to(image_tag(download_url, :alt => @attachment.display_name), download_url) %>
+  <% elsif @attachment.content_type && @attachment.content_type.match(/\Avideo\/|audio\//) %>
+    <% js_bundle :file_preview %>
+    <h2><%= @attachment.display_name %></h2>
+    <div id="media_preview" <%= media_preview_attributes(@attachment) %>></div>
+    <span style="font-size: 1.2em;">
+        <%= link_to "Download #{@attachment.display_name}", download_url %>
+    </span> (<%= @attachment.readable_size %>)
   <% else %>
     <% js_bundle :file_show %>
     <h2><%= @attachment.display_name %></h2>