它可能是
data:image/jpeg;base64,
data:image/png;base64,
data:image/gif;base64,
replace(/^.*?,/,'')
/^data:image\/(jpeg|png|gif);base64,//^data:image/(jpeg|png|gif);base64$/g
更通用点的
('data:image/jpeg;base64,').replace(/data:image\/([^;]+).*/i,'$1');//jpeg