Hello. I'm trying to start on the Build A Data Model for a Recruiting App Project. https://trailhead.salesforce.com/projects/build-a-data-model-for-a-recruiting-app/steps/create-a-custom-object-for-reviews It asks that I install an unmanaged package but when i click on the link provided it says the app cannot be installed because the package is not found. 
"The requested package doesn't yet exist or has been deleted. If this is a recently created package version, please try again in a few minutes or contact the package publisher."
I need to understand roll-up fields (starting from the basics) so i need to complete this or is there another, newer module that will explain it better?

Hi Experts,

I know that this is not a standard functionality. However, can anyone share a sample code for sending email alert when a new attachment has been added to the custom object.

Here is the specific scenario:

When a new file or multiple files has been added for a custom object, I would like a specific set of users to be notified.

Any assistance is greatly appreciated. Thanks.