Missed a rogue _ before the file extension, not sure why the dev has it there.
@ -1,5 +1,5 @@
class WimowehBeta < Cask
url 'http://www.serialangels.co.uk/sa-content/uploads/2013/12/Wimoweh.0.66.BETA.zip'
url 'http://www.serialangels.co.uk/sa-content/uploads/2013/12/Wimoweh.0.66.BETA_.zip'
homepage 'http://www.serialangels.co.uk/index.php/wimoweh/'
version '0.66-beta'
sha1 '5d408c963e0b4318cbbe933e17facde8dad74a51'