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I get file attachments from both friends and strangers. Should I just open the ones from friends?
I would even think twice before opening file attachments from friends. What if your friend's computer has been infected by a virus, and the virus emails you a message with itself attached? You get the email, think it's from your friend, and open the attachment. *Poof!* Your computer is now infected!
The only time you should open an attachment even from a friend is if it is a file type that can't carry viruses (generally speaking you can open image files, but even with those, believe it or not, someone developed a virus to attach to an image (read below for a question and answer about that).
If you have any doubt about whether a file type can carry a virus, don't open the email. Call or email your friend and say "Did you send me this file?" If they say no, obviously you aren't going to open it. And if the message is from a stranger? Don't open it at all! I occasionally get resumes from people who want to work for me, and rather than mailing them, they send them as Microsoft Word Documents. I don't know them from Adam, and I won't open their attachments.
I get emails with picture files (stuff with .gif?x=5, .bmp, and .jpg?x=5 file extensions) attached. I know there are some kinds of email attachments I should never open; what about these? Can I get infected through these?
Well...yes, technically, it is possible, but only under the following scenario. Your computer has to already be infected with another virus whose purpose is to act as interpreter for the image virus. Seems kind of pointless to me. (The "Perrun Virus" was an example of this kind of virus, which attached to jpg files. A lot of hype, not much action)
However, you still need to be careful when opening images attached to emails. Be sure that the file extension really is an image extension (the ones you listed, plus a few others). I've seen emails come in with file names like "yourpicture.jpg?x=5.exe". Clever. The extension isn't really 'jpg', it's 'exe', and you already know you should never open those. And remember: if the attachment is from someone you don't know, why would you bother opening it anyway?