Revert "MM-27312 Allow uploading 10 files per post (#16235)" (#16908)

This reverts commit 8e2986e5ed.

i18n/uk.json

@@ -4557,7 +4557,7 @@
   },
   {
     "id": "model.post.is_valid.file_ids.app_error",
-    "translation": "Invalid file ids. Note that uploads are limited to 10 files maximum. Please use additional posts for more files."
+    "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",