typically code like this:

let mut file = File::open("bigfile").await.unwrap();
let mut vec = Vec::new();
file.read_to_end(&mut vec).await?;
// ..snippet
op.write("file", vec).await?;

it will load the file into memory at first, but
reqwest-0.11.15/src/async_impl/body.rs:218 originally support stream upload

I test different ways to upload a 1GB file, and the process usage:

using stream:

not using stream: