Select By Type
The Select By Type shell extension makes it simple. Just select one or more files and/or folders in the usual way, right-click any selected item and choose Select By Type from the context menu. Any file with the same extension as one of those you selected will automatically become selected itself, along with any folders if you selected a folder.
In the screenshot, several files have .html extensions. Finding them all is as simple as right-clicking one and choosing Select By Type. (Need to change those extensions? It's easy using Batch Rename. Or move them to a different folder or drive using Copy/Move To Folder.)