In order to support seamlessly both application/json and multipart/form-data requests, the following normalization rules are applied if the json field is a plain string:

• "true" is converted to the json true
• "false" is converted to the json false
• "null" is converted to the json null
• "[1,2,3]" is converted to the json [1,2,3]
• {'"{"a":1,"b":2}"'} is converted to the json {'{"a":1,"b":2}'}
• numeric strings are converted to json number
• double quoted strings are left as they are (aka. without normalizations)
• any other string (empty string too) is double quoted

Alternatively, if you want to avoid the string value normalizations, you can wrap your data inside an object, eg.{'{"data": anything}'}