Add file function to async_impl::multipart #2110

Job	Run time
Check Style	5s
MSRV	3m 35s
Android	19s
linux / nightly	1m 27s
linux / stable	37s
unstable features	1m 12s
linux / beta	1m 7s
WASM	43s
macOS / stable	1m 24s
windows / stable-x86_64-msvc	3m 43s
windows / stable-i686-msvc	3m 13s
windows / stable-x86_64-gnu	6m 45s
windows / stable-i686-gnu	7m 0s
feat.: default-tls disabled	44s
feat.: rustls-tls	51s
feat.: rustls-tls-manual-roots	44s
feat.: rustls-tls-native-roots	35s
feat.: rustls-tls-no-provider	23s
feat.: native-tls	55s
feat.: default-tls and rustls-tls	56s
feat.: rustls-tls and rustls-tls-no-provider	39s
feat.: cookies	57s
feat.: blocking	1m 9s
feat.: blocking only	1m 3s
feat.: gzip	56s
feat.: brotli	55s
feat.: deflate	57s
feat.: json	51s
feat.: multipart	57s
feat.: stream	49s
feat.: socks/default-tls	44s
feat.: socks/rustls-tls	39s
feat.: hickory-dns	1m 8s
feat.: zstd	47s
Docs	1m 35s
CI is green	1s
	50m 25s

Provide feedback