fix(isMimeType): also match underscores

fixes a few valid MIME types not getting matched

fixes validatorjs#2119

src/lib/isMimeType.js

@@ -26,7 +26,7 @@ import assertString from './util/assertString';
 // NB :
 //   Subtype length must not exceed 100 characters.
 //   This rule does not comply to the RFC specs (what is the max length ?).
-const mimeTypeSimple = /^(application|audio|font|image|message|model|multipart|text|video)\/[a-zA-Z0-9\.\-\+]{1,100}$/i; // eslint-disable-line max-len
+const mimeTypeSimple = /^(application|audio|font|image|message|model|multipart|text|video)\/[a-zA-Z0-9\.\-\+_]{1,100}$/i; // eslint-disable-line max-len
 
 // Handle "charset" in "text/*"
 const mimeTypeText = /^text\/[a-zA-Z0-9\.\-\+]{1,100};\s?charset=("[a-zA-Z0-9\.\-\+\s]{0,70}"|[a-zA-Z0-9\.\-\+]{0,70})(\s?\([a-zA-Z0-9\.\-\+\s]{1,20}\))?$/i; // eslint-disable-line max-len