This is a limitation of windows - that having the file open in one process (Jetty has mapped the file to it's cache) means that you can't write that file with another process.
You can turn off caching in the default servlet by setting <useFileMappedBuffer> to false in webdefault.xml. This file may be found in the etc subdirectory of Jetty. If you are using the maven-jetty-plugin, you'll have to specify its directory location in the pom.xml.
This problem is related to the Troubleshooting entry Files locked on Windows.