actix-files: Firefox cannot stream video provided by a `NamedFile`

[aurakle]

While streaming a video, Firefox may make requests with Range: 0-. It is expected that the server will respond with Transfer-Encoding: chunked. However, actix-files does not provide an option for chunked responses. As a result, video playback behaves unexpectedly, as Firefox attempts to download the entire file before continuing. This issue, as far as I can find, was caused by #1496 and has existed since. While reverting it would solve this issue, that would certainly result in someone else being in a similar position as I am in the future*. Instead, I would advise that there be a method of configuring it, explicitly, to use chunked encoding.

*Though, to be fair, all HTTP 1.1 clients are per-spec supposed to support chunked encoding. (Unless I'm reading this wrong.)