You can explicitly set the content-type I believe, otherwise the mime-type gem will try to guess based on the file extension of the thing you are uploading (so if it has no extension it will be binary/octet-stream). My guess is that you are probably using files with no extension, so you would need to either add the extension or explicitly tell carrierwave that they are image/png
Let me know if that doesn't work out and we can dig in more or try something else.
Thanks!
wes