We get the following error in MS Flow when trying to create Email attachments. Per Alice Zhang at MS Support - tried and got the same error.
Create a new record in Dynamic 365 don’t support entities of type 'attachment' in Microsoft Flow currently.
If you would like this feature to be added in Microsoft Flow, please submit an idea to Flow Ideas Forum:
This idea is duplicate to https://powerusers.microsoft.com/t5/Flow-Ideas/Attach-File-to-Dynamics-365/idi-p/143680
@LauraWare there is a workaround solution using a combination of the CDS connector and CDS (current environment) connector.
But it will definitely help if the Attachment lookup field of the ActivityMimeAttachment entity is not mandatory and we can just create it in one step using CDS (current environment) connector.