Function extract_form_file_body 

pub fn extract_form_file_body<T: StreamReader>(
    reader: &mut T,
    body_buffer: &mut Vec<u8>,
    boundary: &String,
    form_part: &mut FormPart,
    form_part_limit: Option<&FormPartLimit>,
) -> Result<FormPartResult, MultipartFormDataError>

It writes the file to temporary file. Example to copy file

// Example to copy temp file

let filename = &form_part.filename.unwrap();
let owned = filename.to_owned();
let path = temp_file.path();

fs::copy(path, owned).expect("Error copying");