[MM-28945] api4/file: add missing return statement (#15552)

* api4/file: add missing return statement

* api4/file: add a test case for GetPublicFile; request a deleted file

* add missing check

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

api4/file.go

@@ -674,6 +674,7 @@ func getPublicFile(c *Context, w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
+		return
 	}
 	defer fileReader.Close()