Hi Beth,
Yes, look in Web.config and you should be able to find them, AllowedUploadFileExtensions and AllowedLessPriveledgedUserUploadFileExtensions are probably the ones you are looking for. The second one I believe is for users who can only upload to a user specific location whereas the first applies to general browse and upload roles.
And some features are limited also for specific things, you'll see some other settings there but it should be pretty obvious what they are for.
You can copy those into user.config so you won't lose customizations during upgrades.
Best,
Joe