By reading code, yes the File leak detector plugin is calling the attach method on a com.sun.tools.attach.VirtualMachine instance which may throw something like
java.io.IOException: Can not attach to current VM
The workaround for this exception is to add "-Djdk.attach.allowAttachSelf=true" in the java command line.
Except that the file leak detector plugin does not attach from the same process. It creates another java process and attaches to the Jenkins VM from there.
So I don't think that the file leak detector plugin will fail on JDK 9. The plugin may not be aware of new methods in JDK 9 to leak files but otherwise, I think it will work.