I know that, if I wanted to grep for a pattern only on files with certain extensions, I could do this:
// searches recursively and matches case insensitively in only javascript files
// for "res" from the current directory
grep -iIr --include=*.js res ./
I've been trying to search for a way to do this via git grep as well (to take advantage of git grep's speed from the indexes it stores with its tree), but to no avail. I saw here that excluding certain file types is not possible.