How to search for files by file type on Windows 8?

On the top right of Windows Explorer, type *.extension in the search box.

i.e. to search txt files you should type:

*.txt

For multiple files use:

*.doc OR *.txt OR *.pdf

This link can be helpful

The usage Video Here

I find Everything very useful in my daily usage. The search is highly flexible due to the usage of wildcards. It will scan all your connected hard drives only once and once it has completed scanning all the searches you perform are super duper fast! To answer your question, your search in Everything would be *.txt or *.csv.