Files on the content tree, although has an ASPX ending (which can be changed in the Settings), it should still render the file as the .mht
If this is downloading vs. rendering, you probably just need to define the proper MIME Type in your IIS, this tells the browser "What" a file is so it can properly handle it.
i think the mime type is multipart/related, try adding that for .mht in your IIS settings.
OTherwise if you must have a .mht extension, you can do this through the URL properties of the file node, create a new Alias and specify mht for the extension.