I made PR and a jira issue:
https://issues.redhat.com/projects/UNDERTOW/issues/UNDERTOW-2552
Perhaps in next versionIf multipart form data contains non ascii 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.