Replace public use of mime crate with &str (#642)

Replaces `Field::content_type`'s return type with `&str`.
This is a breaking change.

Closes #637

examples/multipart-form/src/main.rs

@@ -65,8 +65,16 @@ async fn accept_form(
 ) {
     while let Some(field) = multipart.next_field().await.unwrap() {
         let name = field.name().unwrap().to_string();
+        let file_name = field.file_name().unwrap().to_string();
+        let content_type = field.content_type().unwrap().to_string();
         let data = field.bytes().await.unwrap();
 
-        println!("Length of `{}` is {} bytes", name, data.len());
+        println!(
+            "Length of `{}` (`{}`: `{}`) is {} bytes",
+            name,
+            file_name,
+            content_type,
+            data.len()
+        );
     }
 }