How can I ignore all files except those with a certain extension in git?

*
!*/
!*.php
!*.css
!*.html
!*.js

For those, who would like to include file extensions which are located in subdirs like @jmborr and @Racso

# .gitignore for excluding all but certain files in certain subdirs

*
!*.cfg
!/**/
!certain/subdir/i_want_to_include/*.cfg

when you exclude everything ('*'), you have to white-list folders ('/**/'), before being able to white-list files.

Found in: https://stackoverflow.com/a/33983585/5388625