Function sub_converter_for_format 

pub fn sub_converter_for_format(format: &str) -> Option<&'static SubConverter>

The converter registered for a format name, which is not an extension: music21’s findSubConverterForFormat, by its class.

use music21::common::formats::sub_converter_for_format;

assert_eq!(sub_converter_for_format("t").unwrap().class_name, "ConverterText");
assert!(sub_converter_for_format("mxl").is_none());