The restriction is set (by default) for a reason. Attachments bring problems. FACT!
The safety feature is set so you don't download attachments, which can and WILL bring viruses. Just because you know the sender doesn't mean that the attachment is safe. Don't do it if you are a beginner!
What is it that you intend to receive? If it is a photo, ask the sender to post the pictures to a free site, such as Yahoo Photos and send you the link. If it is a document, ask the sender to copy the text and paste it into the e-mail.
If you are a beginner, don't ask how to turn off the safety feature that will protect you...from yourself!
Sorry to be so blunt, but once people start learning how to download attachments, they usually come back and ask "What happened to my computer!"
Better safe than sorry!