Tomcat gives you a http request object where you can just grab the input stream object and pass it to pretty much every library that processes files because opening a file just gives you a fileinputstream so adding general support for inputstreams is much easier than actually adding support that only works on files.

It's not at all unlikely, this is the default behavior for various setups, e.g. nodejs with express which is primarily a streams based system where you'd have to do extra work to write to disk.