uri = URI('http://127.0.0.1:8080')
http = Net::HTTP.new(uri.hostname, uri.port)
http.response_body_encoding = true
http.send_request('GET', uri.request_uri)

/opt/homebrew/lib/ruby/gems/4.0.0/gems/net-http-0.9.1/lib/net/http/response.rb:381:in 'String#force_encoding': unknown encoding name - "UTF-8" (ArgumentError)
 @body.force_encoding(enc) if enc
	from /opt/homebrew/lib/ruby/gems/4.0.0/gems/net-http-0.9.1/lib/net/http/response.rb:381:in 'Net::HTTPResponse#read_body'

$ curl -I http://127.0.0.1:8080/
Content-Type: text/html; charset="UTF-8"
Server: WEBrick/1.9.2 (Ruby/4.0.1/2026-01-13)

irb> Encoding.list.flat_map { it.name.delete('0-9A-Za-z').chars }.tally.sort_by(&:first).to_h
=> {"-" => 86, "_" => 1}

$ git diff
diff --git a/lib/webrick/httpservlet/filehandler.rb b/lib/webrick/httpservlet/filehandler.rb
index 4e759d8..2de7785 100644
--- a/lib/webrick/httpservlet/filehandler.rb
+++ b/lib/webrick/httpservlet/filehandler.rb
@@ -489,3 +489,3 @@ module WEBrick
 else
- type << "; charset=\"#{enc.name}\""
+ type << "; charset=#{enc.name}"
 end

http.send_request('GET', uri.request_uri)
# #<Net::HTTPOK 200 OK readbody=true>