diff --git a/unrouted_handler.go b/unrouted_handler.go
index 3769bdf..675519b 100644
--- a/unrouted_handler.go
+++ b/unrouted_handler.go
@@ -456,6 +456,17 @@ func (handler *UnroutedHandler) PatchFile(w http.ResponseWriter, r *http.Request
 		return
 	}
 
+	if info.SizeIsDeferred && r.Header.Get("Upload-Length") != "" {
+		uploadLength, err := strconv.ParseInt(r.Header.Get("Upload-Length"), 10, 64)
+		if err != nil || uploadLength < 0 {
+			handler.sendError(w, r, ErrInvalidOffset)
+			return
+		}
+
+		info.Size = uploadLength
+		info.SizeIsDeferred = false
+	}
+
 	if err := handler.writeChunk(id, info, w, r); err != nil {
 		handler.sendError(w, r, err)
 		return