Proxy returns 400 error

When I try using this I get the following error:
```xml
<?xml version='1.0' encoding='UTF-8'?><Error><Code>InvalidArgument</Code><Message>Invalid argument.</Message><Details>POST object expects Content-Type multipart/form-data</Details></Error>"```

If it's any relevance I am using `api-eu.mixpanel.com` instead of `api.mixpanel.com`