in commons-fileupload2-core/src/main/java/org/apache/commons/fileupload2/core/MimeUtils.java [252:261]
private static String javaCharset(final String charset) {
// nothing in, nothing out.
if (charset == null) {
return null;
}
final var mappedCharset = MIME2JAVA.get(charset.toLowerCase(Locale.ENGLISH));
// if there is no mapping, then the original name is used. Many of the MIME character set
// names map directly back into Java. The reverse isn't necessarily true.
return mappedCharset == null ? charset : mappedCharset;
}