File attachment in the card (#4053)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

server/model/clientConfig.go

@@ -22,4 +22,8 @@ type ClientConfig struct {
 	// The server feature flags
 	// required: true
 	FeatureFlags map[string]string `json:"featureFlags"`
+
+	// Required for file upload to check the size of the file
+	// required: true
+	MaxFileSize int64 `json:"maxFileSize"`
 }