This is the "...-privatekey.p12" file associated with the service account credential in the Google Cloud Console. A new private key may be generated for a service account if you no longer have the original.

To create a new service account:

• Select your project from the list.
• Navigate to "APIs & Auth" and then "Credentials"
• Click the big red "CREATE NEW CLIENT ID" button
• Select "Service account" and click "Create Client ID".
• A dialog should pop-up to download the private key file for the new service account, save this in a secure location. This is the file you must upload here.

To generate a new key for an existing service account, follow the above instructions until the "CREATE NEW CLIENT ID" step and instead find the Service Account for which you need a new key and click "Generate new key".