Send e-mail notificacion ( including attached files ) after user send information with Guest Entries
Instead of using the Postmaster plugin, which appears abandoned and hasn't been updated since 2016, you could write a simple plugin that listened to the afterSaveEntry event.

That plugin could grab any info from the submitted Entry model, including uploads/attachments, and manually compose and send an email out.

You can see an example of how to do that in code in the Contact Form plugin.