MM-13559 Update model.post.is_valid.file_ids.app_error text per report (#10055)

Ticket: https://mattermost.atlassian.net/browse/MM-13559
Report: https://github.com/mattermost/mattermost-server/issues/10023

i18n/en.json

@@ -4636,7 +4636,7 @@
   },
   {
     "id": "model.post.is_valid.file_ids.app_error",
-    "translation": "Invalid file ids"
+    "translation": "Invalid file ids. Note that uploads are limited to 5 files maximum. Please use additional posts for more files."
   },
   {
     "id": "model.post.is_valid.filenames.app_error",