RE: Send mail with attachment in D365
Yes, it is possible. I implemented similar requirement.
1. You need to put custom button a ribbon - Ribbon Bench for putting a custom button, call a javascript from a button.
2. Security based visibility of the button - If something you would like to.
3. Click on that button - Create a Javascript which can call an Action - Research on how to call an "Action" from Javascript.
4. Action Input parameters will be RecordID of that Entity and CWF.
4. Create custom workflow activity which will Read Values of that Record from which the Action was called.
5. In the CWF, after getting the Record GUID, query Annotation entity, fetch the Attached excel file, if more than one attched filter by FirstorDefault().
6. You can use email template stored in CRM if not create your on email body and other required things and send it out.
All you need is a Custom Ribbon, Action and Custom Workflow Activity.
Thanks.