Hello,
Thank you for reaching out. The Email V2 destination step requires the file to be a base64 string.
After the combine step and Excel Writer step, Add a Byte group step.
The Byte Group step will take in Bytes and Output a Base64 string. Specify a field name to return output too.
In the Email V2 destination step config, Specify the content as {@record.FieldName}
Refer to the article below for more information:
How to send multiple attachments in a single email using Email V2 - FAQs / Flows - Synatic Community
If you have any further questions, Please let us know.