Improved send_file.rs example to use streaming.

[acedogblast]

This is to show what I have wrote so far. It does not compile in the current state.
This addresses issues #3432

[seanmonstar]

So far I noticed these two changes.

[acedogblast]

We still have build errors:
line 82 mismatched types
expected StreamBody<_>, found ReaderStream<File>

and line 61 mismatched types
expected Response<BoxBody<Bytes, Error>>, found Response<BoxBody<_, Infallible>>

[seanmonstar]

You'll need to use map_err(|e| e as _) on the Full, and you'll need to import StreamExt and use .map_ok(Frame::data) on the ReaderStream before it can be converted into a StreamBody.

[acedogblast]

I am not sure what you mean by map_err(|e| e as _) on the Full .
Also are you referring to StreamExt from the tokio-stream crate(https://docs.rs/tokio-stream/latest/tokio_stream/trait.StreamExt.html)?

[seanmonstar]

I've tested out all the rest of the changes I'm suggested, and it works at the end.

[acedogblast]

I can also confirm that it builds and works. Now ready to pull.