
Currently, Get attachment Graph API can get an attachment that is attached to a user event, message, or group post. It does not support getting an attachment that is attached to list item.
As a workaround, you could use REST API to get the attachment that is attached to list item.
GET https://{tenant}.sharepoint.com/_api/web/lists/getbytitle('{list-title}')/items({item-id})?$expand=AttachmentFiles
Meanwhile, to get list attachments by Graph API, I recommend you give suggestion in the feedback portal. Many features of current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding and support.
References:
https://www.billyperalta.com/Accessing%20SharePoint%20REST%20API%20using%20Postman/
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