Add file function to async_impl::multipart #2103

Job	Run time
Check Style	7s
Android	23s
MSRV	3m 9s
linux / nightly	1m 25s
linux / stable	44s
unstable features	1m 10s
WASM	1m 0s
linux / beta	1m 14s
macOS / stable	54s
windows / stable-x86_64-msvc	3m 11s
windows / stable-i686-msvc	3m 1s
windows / stable-x86_64-gnu	3m 4s
windows / stable-i686-gnu	3m 6s
feat.: default-tls disabled	45s
feat.: rustls-tls	59s
feat.: rustls-tls-manual-roots	51s
feat.: rustls-tls-native-roots	44s
feat.: rustls-tls-no-provider	33s
feat.: native-tls	57s
feat.: default-tls and rustls-tls	54s
feat.: rustls-tls and rustls-tls-no-provider	30s
feat.: cookies	1m 4s
feat.: blocking	1m 1s
feat.: blocking only	52s
feat.: gzip	52s
feat.: brotli	54s
feat.: deflate	1m 19s
feat.: json	57s
feat.: multipart	48s
feat.: stream	45s
feat.: socks/default-tls	44s
feat.: socks/rustls-tls	46s
feat.: hickory-dns	1m 3s
feat.: zstd	51s
Docs	0s
CI is green	0s
	40m 37s

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