Receiving 403 when sending large file

When I try to submit a 38 MB file, the API responds with a 403 “Forbidden” with the following error message:

<html>
  <head>
	  <title>403 Forbidden</title>
  </head>
  <body bgcolor="white">
    <center>
      <h1>403 Forbidden</h1>
    </center>
  </body>
</html>

What does this mean? I verified that my API key is correct.

The /files endpoint has a 32 MB file size limit. In order to enforce this limit, requests that try to send files greater than 32 MB will be blocked by the firewall before they can reach the service to save bandwidth. The firewall message isn’t configurable, hence the vague error message.