diff --git a/internal/uploads/validate.go b/internal/uploads/validate.go
index d2e4f49..4251026 100644
--- a/internal/uploads/validate.go
+++ b/internal/uploads/validate.go
@@ -34,6 +34,8 @@ const (
 	ReasonTooLong    Reason = "too_long"
 	ReasonTooLarge   Reason = "too_large"
 	ReasonUnreadable Reason = "unreadable"
+	// ReasonBusy: the queue is full. Not about the file: the user retries later.
+	ReasonBusy Reason = "busy"
 )
 
 // RejectError is a synchronous, user-presentable refusal. Message is written for