Undertow multi-part upload can potentially leak file descriptors

Multiple users have reported that WildFly (13.x and even latest released 15.0.0) version leaks file handles to files that look like:

/opt/wildfly-15.0.0.Final/standalone/tmp/digital-archive.war/undertow449317228871190919upload (deleted)

A brief look at the Undertow code and many different manual upload tests, shows that there is (at least one) code path that can lead to such leaks. A specific manual test does reproduce this leak in

io.undertow.server.handlers.form.FormData.FileItem#write

where if a call to Files.move(...) fails for whatever reason, a Files.copy is attempted and while doing so a

InputStream

is opened but never closed.

The forum reference has more details but it hasn't yet been narrowed down to some reproducible example.