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Hi. I've tried to send fax via sandbox here https://developers.ringcentral.com/api-reference/Fax/createFaxMessage

It was ok, response code 200

The I tried the curl text which was displayed near this method and it return message "Bad request". Here is curl:

curl --request POST --url 'https://platform.devtest.ringcentral.com/restapi/v1.0/account/~/extension/~/fax' --header 'accept: application/json' --header 'authorization: Bearer <mykey> '' --header 'content-type: multipart/form-data' --data '{"attachment":"data:text/plain;name=test.txt;base64,VGVzdCBtZXNzYWdl","to":["12013510654"]}'

Tried to send sms via curl and I succeeded. Seems like I have issues only with fax endpoint