A little trick I learned when sending an email that contains an attachment: add the attachment first before you even begin typing, so you won't forget to add it when you hit send.

Also, before you send, click on the attachment from the email to make sure it's the correct file and that it opens correctly.

Check the size of your attachment. You don't want to send huge files that will clog up someone's mailbox. Many mail programs have a limit of 5MB so if your attachments is bigger, the email may not even send.