fix(isDataURI): add underscore to RegExp

also match valid MIME types with an underscore in their name

fixes validatorjs#2121

src/lib/isDataURI.js

@@ -1,6 +1,6 @@
 import assertString from './util/assertString';
 
-const validMediaType = /^[a-z]+\/[a-z0-9\-\+\.]+$/i;
+const validMediaType = /^[a-z]+\/[a-z0-9\-\+\._]+$/i;
 
 const validAttribute = /^[a-z\-]+=[a-z0-9\-]+$/i;