Add a note when using expand_event_list_from_field, content_type …

…is required to set to `application/json` (#39777) (#39904)

x-pack/filebeat/docs/inputs/input-aws-s3.asciidoc

@@ -120,7 +120,7 @@ characters. This only applies to non-JSON logs. See <<_encoding_3>>.
 ==== `decoding`
 
 The file decoding option is used to specify a codec that will be used to
-decode the file contents. This can apply to any file stream data.  
+decode the file contents. This can apply to any file stream data.
 An example config is shown below:
 
 [source,yaml]
@@ -131,17 +131,17 @@ An example config is shown below:
 Currently supported codecs are given below:-
 
     1. <<attrib-decoding-parquet,Parquet>>: This codec decodes parquet compressed data streams.
- 
+
 [id="attrib-decoding-parquet"]
 [float]
 ==== `the parquet codec`
 The `parquet` codec is used to decode parquet compressed data streams.
 Only enabling the codec will use the default codec options. The parquet codec supports
-two sub attributes which can make parquet decoding more efficient. The `batch_size` attribute and 
+two sub attributes which can make parquet decoding more efficient. The `batch_size` attribute and
 the `process_parallel` attribute. The `batch_size` attribute can be used to specify the number of
-records to read from the parquet stream at a time. By default the `batch size` is set to `1` and 
-`process_parallel` is set to `false`. If the `process_parallel` attribute is set to `true` then functions 
-which read multiple columns will read those columns in parallel from the parquet stream with a 
+records to read from the parquet stream at a time. By default the `batch size` is set to `1` and
+`process_parallel` is set to `false`. If the `process_parallel` attribute is set to `true` then functions
+which read multiple columns will read those columns in parallel from the parquet stream with a
 number of readers equal to the number of columns. Setting `process_parallel` to `true` will greatly
 increase the rate of processing at the cost of increased memory usage. Having a larger `batch_size`
 also helps to increase the rate of processing. An example config is shown below:
@@ -162,6 +162,8 @@ value can be assigned the name of the field or `.[]`. This setting will be able
 the messages under the group value into separate events. For example, CloudTrail
 logs are in JSON format and events are found under the JSON object "Records".
 
+NOTE: When using `expand_event_list_from_field`, `content_type` config parameter has to be set to `application/json`.
+
 ["source","json"]
 ----
 {