Hello @andrie - Yes, unfortunately, multipart form-data is not supported at this time in the APIM dev portal: https://github.com/Azure/api-management-developer-portal/issues/316
With that said, it looks like the API operations themselves will work when OpenAPI v3 spec is used to import the definitions: https://stackoverflow.com/questions/63143532/how-to-send-post-form-data-with-file-and-data-using-azure-api-management
If you opt to self-host your Developer Portal, you can probably customize per your requirement and override this limitation.