Avoid asking the cloud if an image exists

#4392

Amazon's DNS is failing at the moment and some users can't access the
admin panel because of this error.

While `exists?` asks the storage server if the file is actually there,
`file?` just checks if we have the file name stored in the database
and the file should be there. It's much faster and less error prone.

app/serializers/api/admin/enterprise_serializer.rb

@@ -62,7 +62,7 @@ def find_match(tag_groups, tags)
   #   #   medium: LOGO_MEDIUM_URL
   #   # }
   def attachment_urls(attachment, versions)
-    return unless attachment.exists?
+    return unless attachment.file?
 
     versions.each_with_object({}) do |version, urls|
       urls[version] = attachment.url(version)