WINMAIL.DAT files typically come from Outlook users on Windows machines. It is due to the way the sender is sending you an email; rather, in the format the sender is sending the email. People like to do silly things with email, like add HTML formatting or rich-text formatting to the body of their email, foregoing plain-text (good ol' text, no bold, no italic, no colors -- just the words the sender wants you to read) for something much more complicated and unpredictable. Here's an older, but still relevant article on the WINMAIL.DAT problem:
http://facstaff.gpc.edu/~jbenson/resource/winmail.htm
My bet is that your sender is trying to do fancy things to their email -- add color to it, a background image, use a different font (death to Comic Sans!), or something equally complicated and detracting from the actual
content of the email. Perhaps they have a ridiculous signature with a picture logo in it, and when you get emails from them, the signature is typically longer than the content of the email... they're not sending "Prius" emails -- they're clogging the intertubes with "Hummer" emails.
I use the rule of thumb that someone who is trying to "glitz up" their email messages by the aforementioned methods is trying to hide deficiencies in the actual words and sentences that the whole email is about.

Kind of like old men driving expensive sports cars, if you know what I mean.
Instruct the sender to turn off rich-text and/or HTML formatting when they send you emails, and re-send the email. Barring that, have the sender simply "zip" the whole Powerpoint presentation, then send you the zipped file. Barring that, have him/her upload the Powerpoint presentation to a place like
http://www.uploading.com/ so that he/she can just email you a like to download the file.