It is not possible to visualize flowcharts in Confluence by simply adding the flowchart to your page: Confluence adds the file as an attachment, but you will not see its content.
If you want to see your flowchart as an image, you need to install plugins.Most of these come with a price tag.
Keep in mind you also need to buy a license for the program in which you create flowcharts. The plug-in only allows integration of the two programs, you cannot draw with it.

Info: https://www.gliffy.com/
Buy @ https://marketplace.atlassian.com/plugins/com.gliffy.integration.confluence/cloud/overview

Buy @ https://marketplace.atlassian.com/plugins/com.cinergix.confluence.plugins.creately.CreatelyConfluence/server/overview

Info: https://www.youtube.com/watch?v=PlcYJ9q1NcU
Buy @ https://marketplace.atlassian.com/plugins/com.lucidchart.confluence.plugins.lucid-confluence/cloud/overview

Buy @ https://marketplace.atlassian.com/plugins/org.swift.confluence.visio/server/overview