[MM-11287] Add support for plus sign and period/dot in custom URL schemes (#9155) (#10852)

model/config.go

@@ -2871,7 +2871,7 @@ func (mes *MessageExportSettings) isValid(fs FileSettings) *AppError {
 
 func (ds *DisplaySettings) isValid() *AppError {
 	if len(ds.CustomUrlSchemes) != 0 {
-		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[a-z][a-z0-9-]*\s*$`)
+		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[A-Za-z][A-Za-z0-9.+-]*\s*$`)
 
 		for _, scheme := range ds.CustomUrlSchemes {
 			if !validProtocolPattern.MatchString(scheme) {