Need additional implementation in a Firefox plugin

I have a firefox plugin with source code that modifies (encrypts) an attachment on Gmail emails before uploading it. I need one more thing implemented in this plugin as mentioned below: When the user uploads an attachment from a different browser without plugin, and then saves the email as a draft, and then opens the same draft in Firefox with plugin enabled, and sends the email, the attachment is unmodified. Instead, the plugin should display a message that the attachment is unencrypted.