AppExchange products are below

https://appexchange.salesforce.com/appxListingDetail?listingId=a0N30000003IzEDEA0

https://appexchange.salesforce.com/listingDetail?listingId=a0N3000000B4BAdEAN

Note:
Currently it is available out of the box in Lightning.

https://success.salesforce.com/ideaView?id=08730000000Bp7O

1. Open a case and click Send an Email button.

2. Click Attach File.

3. We can select only one file at a time. So, select a file and click Attach to Email button. Add multiple files like this and click Done button.

4. Click Done and view the email.