Error when uploading Plugin to a Container Field on a Mac

[hollyheadhunter]

In preparation for using Filemaker 12's new function "Install Plug-In File", in our current Filemaker 11 database, we created a new table with container fields for our plugins. 

We inserted the plugins for a PC with no problem but when we tried to insert a plugin to a container field on a Mac, Filemaker gave the error: "This file cannot be inserted because it contains other files inside it"
 

We are not trying to insert the whole folder -- just the plugin itself.  How can we insert a plugin to a container field on a Mac?