This eliminates the need for Workhorse to set `Content-Type` by guessing
based on file extension.

If `Content-Disposition` is `attachment`, the browser will prompt the
user for an action. In Chrome, the user is always prompted to save the
file, and the `filename` extension in the `Content-Disposition` header
determines the saved file's default extension. Chrome appears to ignore
`Content-Type` if `Content-Disposition` is an attachment.

In Firefox, the user is prompted with two options: save the file, or
open it with a program. Again, the `filename` extension in the
`Content-Disposition` header determines the saved file's default
extension. In addition, this extension is used to guess which default
program should open the file. If the extension isn't present, Firefox
falls back to `Content-Type`.

Safari combines both approaches. Like Chrome, it only prompts the user
to save the file. Like Firefox, it falls back to the extension provided
in `Content-Type` if the `Content-Disposition` filename is not present.

Hence, Workhorse's setting of `Content-Type` based on extension provides
no additional information and may even be a security risk if the
`Content-Disposition` is subverted.

Changelog: changed