in lib/src/multipart_request.dart [133:141]
String _headerForFile(MultipartFile file) {
var header = 'content-type: ${file.contentType}\r\n'
'content-disposition: form-data; name="${_browserEncode(file.field)}"';
if (file.filename != null) {
header = '$header; filename="${_browserEncode(file.filename!)}"';
}
return '$header\r\n\r\n';
}