docs: Add details for JSON file mutations with curl.

A user asked for more clarification in the docs about this section to avoid
errors related to the format of their JSON file.

wiki/content/mutations/index.md

@@ -779,6 +779,58 @@ Mutation with a JSON file:
 curl -H "Content-Type: application/json" -X POST localhost:8080/mutate?commitNow=true -d @data.json
 ```
 
+where the contents of data.json looks like the following:
+
+```json
+{
+  "set": [
+    {
+      "name": "Alice"
+    },
+    {
+      "name": "Bob"
+    }
+  ]
+}
+```
+
+The JSON file must follow the same format for mutations over HTTP: a single JSON
+object with the `"set"` or `"delete"` key and an array of JSON objects for the
+mutation. If you already have a file with an array of data, you can use `jq` to
+transform your data to the proper format. For example, if your data.json file
+looks like this:
+
+```json
+[
+  {
+    "name": "Alice"
+  },
+  {
+    "name": "Bob"
+  }
+]
+```
+
+then you can transform your data to the proper format with the following `jq`
+command, where the `.` in the `jq` string represents the contents of data.json:
+
+```sh
+cat data.json | jq '{set: .}'
+```
+
+```
+{
+  "set": [
+    {
+      "name": "Alice"
+    },
+    {
+      "name": "Bob"
+    }
+  ]
+}
+```
+
 ## Upsert Block
 
 The Upsert block allows performing queries and mutations in a single request. The Upsert