Did you try:
GET /users/{id | userPrincipalName}/messages/{id}/attachments/{id}/$value
https://learn.microsoft.com/en-us/graph/api/attachment-get?view=graph-rest-1.0&tabs=http
you can use graph explorer to try:
https://developer.microsoft.com/en-us/graph/graph-explorer
first get message:
that returns the id you can use for next call:
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