Could you explain the purpose?
Anyway here's one assuming you support few image types.
(/\.(gif|jpe?g|tiff?|png|webp|bmp)$/i).test(filename)
I have put the whole regular expression within parentheses () so as to disambiguate between the slash (/) operator and RegExp object. See JSLint for more details.
Here's the raw regex as well.
/\.(gif|jpe?g|tiff?|png|webp|bmp)$/i
This Regex also assumes that you're including the dot before the extension. Remove the \. if you're not.
