How Search File Content in Windows 8 File Explorer?

You could try this :

In Explorer

1- Menu View
2- Click Options
3- Click Search tab
4- Select "Always search file name and contents"
5- Click "OK"

Agreed, MS broke something that worked. But try this in the search window adjacent to the file path window. The following searches all .docx files for the string 'fish'.

*.docx content:fish

I haven't figured out how to make it case sensitive or whole words only but better than nothing. I am sure there are other arguments besides 'content' like 'date', 'size' but haven't found the list.

Works for Windows8, not sure about Windows10