getHeaders method
Implementation
@override
Map<String, List<String>> getHeaders() => {
if (length != null) HttpHeaders.contentLength: [length.toString()],
HttpHeaders.contentType: [contentType],
if (nullOrEmpty(fileName))
'content-disposition': ['${disposition.name}'],
if (!nullOrEmpty(fileName))
'content-disposition': ['${disposition.name};filename="$fileName"'],
};