Hello,
The refresh token is used if the server returns a refresh token in the JSON Access Token response, you can check if Dropbox server returns a refresh token, just search for the fieldname: "refresh_token". If this field exists, the refresh token will be used. You can enable the log file in the client and check if this field is returned.
Refresh token is not mandatory so some servers doesn't implement this feature.
Find below an example of response with refresh token (from Google servers)
{
"access_token": "ya29.a0AX9GBdXsdfffhDsF8RyU3wyeDj3rxvGwxQTLz8N20kTf6a9yf3rRNLi7e-Xp86JGiZ7qhqagA3m1jRFQHk7_-I5MsDFgmd4HFdHpPoip6FNTgfLIP6lVQy9vOVssm9g6VHvCI2LcRkMbcbuRiJGQAUWa-vOaCgYKAX8SARESFQHUCsbCQbQK9HEvWpV4jcSkWJxCYA0163",
"expires_in": 3599,
"refresh_token": "1//03gHdzwIFl08SCgYIARAAGAMSNss-L9IrQko6NSkwaAVUWI9Zcdo0x9Peeq9M_oqkuM2hcWaMP1SucuEmcYAKj-8v59ECv0",
"scope": "https://mail.google.com/",
"token_type": "Bearer"
}
Kind Regards,
Sergio