Just like your choice of language in email and the frequency with which you use emoticons says a lot about you, so does the the format in which you send your attachments.
Randall Munroe, as ever, articulates that best in today's XKCD. The .TEX file — the preserve of academics and computer scientists — surely must rank top. But others should fare well, too, as Munroe points out in the alt text of the image:
"I have never been lied to by data in a .txt file which has been hand-aligned."
Hear, hear. There's something about a PDF that inspires confidence, too: a sense of professionalism. GIFs, on the other hand, are the preserve of those with a puerile sense of humour. Yes, us included. But do you agree with the entire chart? [XKCD]
Note: This image is a .PNG. Make of that what you will.