Reference: Tree Structure View in IntelliJ SDK Docs

The Tree Structure Provider sample project implements com.intellij.treeStructureProvider Extension Point, which allows modifying the structure of the project tree in the Project View panel. This implementation limits the presented files to the Plain Text files only.

The current implementation checks if a Project View node represents a directory or file of the PlainTextFileType type. Otherwise, an element is not included in the results list, so only directories and plain text files are rendered.

Reference: Plugin Extension Points in IntelliJ SDK Docs