Client: I need some images converting to jpeg, can you do that for me?
Me: Sure, just email them to me and I'll do it for you.
Emails arrive and they are jpegs, so I ring client back.
Me: Hi, these are jpegs already, did you need them converting to cmyk or
Client: No, they just need to be jpegs.
Me: OK, well, do you see the file extension is .jpeg?, that tells you they are jpegs.
Client: That's great, do you think you could email them back to me?
Me: Err... yes.