A quick Apex code snippet for inserting an Attachment for use in a testMethod. This can be useful to ensure sufficient code coverage in classes that depend on existing attachments.
/**
* Create a testing attachment for the opportunity
*/
private static void addAttachmentToParent(Id parentId) {
Blob b = Blob.valueOf('Test Data');
Attachment attachment = new Attachment();
attachment.ParentId = parentId;
attachment.Name = 'Test Attachment for Parent';
attachment.Body = b;
insert(attachment);
}