Update fabricator for MediaAttachment to attach a file according to type (#3862)

This fixes a random spec failures since commit
d55f2072.

spec/fabricators/media_attachment_fabricator.rb

 Fabricator(:media_attachment) do
   account
-  file { [attachment_fixture(['attachment.gif', 'attachment.jpg', 'attachment.webm'].sample), nil].sample }
+  file do |attrs|
+    [
+      case attrs[:type]
+      when :gifv
+        attachment_fixture ['attachment.gif', 'attachment.webm'].sample
+      when :image
+        attachment_fixture 'attachment.jpg'
+      when nil
+        attachment_fixture ['attachment.gif', 'attachment.jpg', 'attachment.webm'].sample
+      end,
+      nil
+    ].sample
+  end
 end