When using the file and trying to get all files in C:\myfolder, the following setup will not work.
The issue seems to come from the use of Dir#glob - https://github.com/jordansissel/ruby-filewatch/blob/master/lib/filewatch/watch.rb#L115
When using Dir.glob, \ is treated as a special character for escaping - http://en.wikipedia.org/wiki/Glob_(programming)#Syntax. The path entered into Logstash appears to be handed in unadultered to FileWatch::Watch, so the Dir.glob call fails to find any results.
The easy fix is to change all paths to unix style, so instead of C:\\myfolder
*.log it becomes C:/myfolder/*.log (which I like more anyway ). However for a Windows user this would be pretty confusing and there is no indication that the file path is wrong.