Verify that the recipient will receive your attachment. Include an appropriate subject line and text in the body of the message, so that the recipient knows what the attachment is. Compress the file (or compress a folder if there are multiple files). Verify that you are attaching the correct version of the file(s).
Do not send an attachment without getting permission from the recipient first. When you do send an attachment, make sure to use an appropriate subject line and at least a little text in the body to make sure that the recipient knows what the attachment is and why you are sending it.
If you receive an e-mail with an attachment, and you do not know who the message is from or why they are sending an attachment, you should not open the attachment. Attachments can contain computer viruses.