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."
Here, here. 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.