Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/platform/services/docextractor/plain.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package docextractor
+
+import (
+	"io"
+	"unicode"
+	"unicode/utf8"
+)
+
+type plainExtractor struct{}
+
+func (pe *plainExtractor) Match(filename string) bool {
+	return true
+}
+
+func (pe *plainExtractor) Extract(filename string, r io.ReadSeeker) (string, error) {
+	// This detects any visible character plus any whitespace
+	validRanges := append(unicode.GraphicRanges, unicode.White_Space)
+
+	runes := make([]byte, 1024)
+	total, err := r.Read(runes)
+	if err != nil && err != io.EOF {
+		return "", err
+	}
+
+	if total == 0 {
+		return "", nil
+	}
+
+	count := 0
+	for {
+		c, size := utf8.DecodeRune(runes[count:])
+		if !unicode.In(c, validRanges...) {
+			return "", nil
+		}
+		if size == 0 {
+			break
+		}
+		count += size
+
+		// subtract the max rune size to prevent accidentally splitted runes at the end of first 1024 bytes
+		if count > total-utf8.UTFMax {
+			break
+		}
+	}
+
+	text, _ := io.ReadAll(r)
+	return string(runes[0:total]) + string(text), nil
+}