I don't know of any JS
Also .NET, by default, will only accept 4mb file uploads unless you explicitly go in and change this through the web.config file.
Another thing to considers: .NET 1.1 has extremely poor memory management when uploading files like this. If say, for example, you were to upload a 40mb file, the file will be passed into the aspnet_wp.exe process where it would then be saved to disk.
aspnet_wp.exe consumes 40mb of memory on the server when that file is passed in and instead of automatically relasing that memory when it is done saving, it only releases a little if at all. What eventually happens is that aspnet_wp.exe consumes 80% of server memory and the process is automatically restarted... which can cause you app to fail.
My suggestion would be to look into an HttpModule/handler which will stream the data straight to disk and bypass loading the file into the worker account.
"The one language all programmers understand is profanity."