There is an requirement to send existing or uploaded docs to external system with metadata , with SFTP protocol I think we cant send metadata , What would be the better apprach
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The depends a lot on the type of interfaces the other system supports. If this is a document management system like OpenText, it probably has some API which you might want to have a look at.
Integrations support Multipart Form Data (multipart/form-data). That should allow you to make the file the first part, and add a JSON object as the second part.
https://docs.appian.com/suite/help/21.4/Integration_Object.html#http-integration-definition