I'd bet that your browser is timing out entirely on its own. Unfortunately, that message it's giving you isn't very useful! If that's one of Internet Explorer's "helpful" errors, you might wish to try it with another browser that reports exactly what the server responded.
You could also take a look in your webserver logs in /var/log/httpd and see what the server is seeing when you try to mail that file.
You might try specifying the upload_max_size in bytes rather than using the 'M' suffix.
Of course, there's something to be said for using something other than email for sending 200MB files around, a practice which may make you unpopular with the sysadmins of any mail servers that message passes through that aren't under your control.

It's certainly outside the envelope that IMP is tested with.
-Rich