From what I can gather, paperclip is runnning the "file" command to identify a file type.
Windows has no such "file" command. Unfortunatly, there is a Jruby 1.7.4 issue with the "current thread" variable which prevents the type_from_file_command from failing gracefully. Here is a work around
@hathawad have you confirmed this is fixed on master for 1.7.5 yet? We did change the logic of this so that we no longer have the current thread issue. I just want to make sure so we can make sure this is addressed for 1.7.5.