Fixes after review

- Maybe it could be rephrased to "attribute name prefixed with X-Attribute-"
- Maybe it worth renaming to userAttributeHeaderPrefix
- Requirement for having only one file uploaded at a time has to be reflected in documentation.
- Maybe this should be reflected in doc
- I'm not sure if missing timestamp attribute should be forced on gateway level. Normally it should be set by app and there should be a way not to set it.

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

app.go

@@ -33,6 +33,8 @@ type (
 
 		jobDone chan struct{}
 		webDone chan struct{}
+
+		enableDefaultTimestamp bool
 	}
 
 	App interface {
@@ -78,6 +80,8 @@ func newApp(ctx context.Context, opt ...Option) App {
 
 	a.hdr = newHeaderFilter(a.log, a.cfg)
 
+	a.enableDefaultTimestamp = a.cfg.GetBool(cfgUploaderHeaderEnableDefaultTimestamp)
+
 	a.wlog = logger.GRPC(a.log)
 
 	if a.cfg.GetBool(cmdVerbose) {