There are no restrictions about the formats or data types your application can use with the API. However, customers often choose to encrypt their data, so the complexity of data-type selection gets obfuscated from the API layer. Even binary data can be encoded and then encrypted.
Developers are encouraged to encode plaintext content in base64. Encoding resolves a difficulty with special characters and escapes conflicts with single and double-quote characters in the payload content.
This website provides an easy-to-use testing environment for manually encoding and decoding content.