Requesting unknown file type returns Ruby object w/ address

How it was found

Hello sec folks,

requesting a report you are not allowed to acccess along with an unhandled filetype extension discloses a Mime::NullType Ruby object representation with a corresponding memory address.

Example: https://hackerone.com/reports/1337.foo

Request:

GET /reports/1337.foo HTTP/1.1
Host: hackerone.com

Response:

HTTP/1.1 401 Unauthorized
....
Content-Type: #<Mime::NullType:0x007f3588fe32c8>; charset=utf-8
...

Cheers!