Learning from here: List attachments and Use query parameters
When I call v1.0/me/messages/{message id}/attachments/?$filter=isInline eq true. It return
v1.0/me/messages/{message id}/attachments/?$filter=isInline eq true
There is a known bug with the /attachments endpoint affecting support the $filter clause.
/attachments
$filter
You can track the status of this issue at GitHub. I've also added a reference to your question here.