An suggestion is to make a new collection, add new rows then the attachments like you would usually do repeat as many times as necessary. Then you use a calculation stage to replace the collection within the collection with the one you have made. So it would be like this:
1. Make Collection ""Attachments""
2. Add new row and value to ""Attachments""
3. Keep doing step 2 until satisfied then move onto step 4
4. Use a calculation stage to store the collection ""Attachments"" in the collection ""Email""
5. If you need to do this on multiple rows in the collection ""Email"", just surround all these steps with a loop of the collection ""Email"".
Good luck and have fun