multer still uses http 0.2 which required us to eagerly convert the headers in multer::Field to an http 1.0 HeaderMap.

A PR to update multer to http 1.0 has been merged but not yet published. When its published we can update https://github.com/tokio-rs/axum/blob/main/axum/src/extract/multipart.rs#L98-L103 and https://github.com/tokio-rs/axum/blob/main/axum-extra/src/extract/multipart.rs#L118-L128 to just reuse the headers in the field.