Fixes after review

- It's confusing. If there is no difference, I suggest having the route named after the protocol verb or resulting handler. So it should be either post or upload.
- Don't you find that it would be more understandable without else ifs?
- Why do we need a temporary file here?
- etc

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

app.go

@@ -181,8 +181,8 @@ func (a *app) Serve(ctx context.Context) {
 	r := router.New()
 	r.RedirectTrailingSlash = true
 
-	a.log.Info("enabled /put/{cid}")
-	r.POST("/put/{cid}", a.upload)
+	a.log.Info("enabled /upload/{cid}")
+	r.POST("/upload/{cid}", a.upload)
 
 	a.log.Info("enabled /get/{cid}/{oid}")
 	r.GET("/get/{cid}/{oid}", a.byAddress)