Extract

Hello Adam, thank you for your great blog, it has been really useful.
As a follow-up, how do you think i can now extract from the MultipartStream

- file_name
- content_type

when i start extracting content before the stream macro i get an error of

`cannot return value referencing local data self.0 `

```
impl MultipartStream {
pub async fn into_stream(
mut self,
) -> impl Stream> {
let field = self.0.next_field().await.unwrap().unwrap();
let file_name = field.file_name().unwrap().to_string();
let content_type = field.content_type().unwrap().to_string();
async_stream::stream! {
for await value in field {
yield value;
}
}
}
}
```

the idea was to return those values as a struct or tuple.

What i can see is that the file_name needs to be retrieved outside of the async_stream::stream! as those attribute are needed before actually streaming.

Thank you!!