Confluence Cloud currently does not index or allow searching the contents of .render and .html file attachments. Multiple investigations confirmed that these file types are not supported by the underlying indexing services. Customers have a business need to search for content within these files, as they often contain important data such as HTML tables or reports. We request support for indexing and searching .render and .html files to improve content discoverability in Confluence.

Currently Supported files - https://developer.atlassian.com/platform/media/advanced-topics/file-previews/#supported-file-types