If multipart form data contains non English character and placed on 1024-1025's byte than undertow cut this character into two part and recognize theese as two character.

Bug is after this pull request:

UNDERTOW-2337 Multipart form-data larger than 16KiB is not available through Servlet getParameter API.

This function handle just ascii string's well:

public static String readFile(InputStream file, Charset charSet) {

try (BufferedInputStream stream = new BufferedInputStream(file)) {

byte[] buff = new byte[1024];

StringBuilder builder = new StringBuilder();

int read;

while ((read = stream.read(buff)) != -1) {

builder.append(new String(buff, 0, read, charSet));

}

return builder.toString();

} catch (IOException e) {

throw new RuntimeException(e);

}

}