EmailMessageTrigger.cls

trigger EmailMessageTrigger on EmailMessage(after insert, after update) {
    Set<Id> emailsWithAttachments = new Set<Id>();

    for(EmailMessage email : Trigger.New){
        if (email.Incoming) {
            emailsWithAttachments.add(email.Id);
        }
    }

    List<Attachment> attachments = [
        select Id
        from Attachment
        where ParentId in :emailsWithAttachments
    ];

    delete attachments;
}