Oh, these answers are way too sophisticated for your situation. It's highly unlikely that your IT department would be using some fancy workaround assuming they know what editor you're using, their point is not to actually mess up your systems. Notepad isn't going to be doing anything with any image or attachment, no matter what, and the text you saw certainly didn't do anything reprehensible to your system. Nor would it have been likely to, even if it had been a real attack, as editors are very personal tools and there are lots and lots of them that'd have to be covered for anything like this to have any sort of detectable effect.

Tell your coworkers to get off your back, and to be more careful themselves. Let me just point out that you dug yourself into this hole though, and achieved the opposite of what was intended... If you hadn't announced that you looked and what you saw, people who clearly don't know a thing about IT security wouldn't be harassing you and feeling superior about it. Instead, some of them would have opened the attachment and seen the message, and gotten a bit chastened themselves. So you've messed up IT's test all the way down the line, emphasizing the Dunner-Kruger effect which is no doubt the cause of sending out these tests. Stop it!

Oh, these answers are way too sophisticated for your situation. It's highly unlikely that your IT department would be using some fancy workaround assuming they know what editor you're using, their point is not to actually mess up your systems. Notepad isn't going to be doing anything with any image or attachment, no matter what, and the text you saw certainly didn't do anything reprehensible to your system. Nor would it have been likely to, even if it had been a real attack, as editors are very personal tools and there are lots and lots of them that'd have to be covered for anything like this to have any sort of detectable effect.

Tell your coworkers to get off your back, and to be more careful themselves. Let me just point out that you dug yourself into this hole though, and achieved the opposite of what was intended…. If you hadn't announced that you looked and what you saw, people who clearly don't know a thing about IT security wouldn't be harassing you and feeling superior about it. Instead, some of them would have opened the attachment and seen the message, and gotten a bit chastened themselves. So you've messed up IT's test all the way down the line, emphasizing the Dunning-Kruger effect which is no doubt the cause of sending out these tests. Stop it!