Is there a way to just commit one file type extension from a folder?

If you want to add all files with a specific file extension, all you have to do is specify it at the time you go to stage the files using a wildcard.

git add *.c

Here .c can be any extension you want.

For me, just git add *.c didn't work. I encountered fatal error like this:

git add *.php
The following paths are ignored by one of your .gitignore files:
wp-config.php
Use -f if you really want to add them.
fatal: no files added

So, in this variant git tried to add ignored files.

But such variant:

git add \*.php

worked flawlessly. By the way, you can do git diff the same way:

git diff -- \*.php