Related Questions:
Drag n Drop into an email
Environment:
Event Temple Web App
Answer:
When attaching files to an email, "Drag and drop" is not currently a feature supported, if you'd like to attach files to an email, please use the paperclip icon.
Additional Information
If attaching the same files regularly to emails, why not upload them into Event Temple & attach them as a link instead? See the below articles for further details on this feature.