In principle so, mixture, but in practice I get e-mails every day from people I know but with attachments I wasn't "expecting". They are usually items for inclusion on the church website, or photographs I might want to use, news about people I know, or other tidbits of information.
If I know the person, and if the file type is "safe", then I open it. None of my regular correspondents sends .exe or .scr and such files.
On the rare occasions (like one a year or so) where the email is bogus/from a hijacked account, Avast usually spots it and flags it before I do. Even then, the body text of the message is patently not from the person it says it is.
I have a neat little add-in to Thunderbird which shows against each e-mail the flag of the country it actually came from. 99% of the legitimate ones show the US or UK flag. If I see Russian, Chinese, Taiwanese, Brazilian etc then I check!