Post Receiver: JSON Payload is not expanded

[christianTF]

The code checks for json and inserts the plain json into a json string.
As the script. itself wraps the content into a json, the plugin only expands the outside json, but not the json wrapped into a string.

The json payload needs to be parsed in the php script and added as object to the json sent to the Gateway.

[christianTF]

The function of receive.php was correctly - json data are expanded as expected.

The special issue arised with the PLEX webhook, that does not send application/json but multipart/form-data, and submits a file (picture) as well as a POST form, where a field is called payload, that included the json.

Therefore, the script did not see a json content (as it is multipart content), and the post variable payload is sent as string to the gateway.

Although time consuming, I now also check the post variables if there content might be a json, and add parsed json also to post variables.
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