You are most likley looking at the wrong trace file. The site is HTTPS so you may have a second trace file that you need to look at.
OR if you dont have SSL scanning on, then you will not be privvy to the media type within the SSL tunnel.
Take a screenshot of the "Top properties" tab, if that says the command name is CONNECT, then this is just the SSL tunnel, not the actual HTTP traffic.
I gave a big long response to something similar in this post:
Thank you for your answer,
I have exclude the drop box appfrom ssl scanning because of problem with the application in transparent proxy.
Later today I will return thessl scanning for the drop box and I will check if I can see the real file type.