🐛 Fixed routes.yaml upload on Ubuntu

no issue

- a specific content type was missing (e.g. Firefox on Ubuntu)
- they send a different content-type to the server: application/x-yaml
- was reported in the forum: https://forum.ghost.org/t/cant-upload-routes-yamls-for-dynamic-routing-error-please-select-a-valid-yaml-file-to-import/2986

core/server/config/overrides.json

@@ -54,7 +54,7 @@
         },
         "routes": {
             "extensions": [".yaml"],
-            "contentTypes": ["text/plain", "text/yaml", "application/octet-stream", "application/yaml"]
+            "contentTypes": ["text/plain", "text/yaml", "application/octet-stream", "application/yaml", "application/x-yaml"]
         }
     },
     "times": {