isDataURI: is not matching valid MIME types with an underscore

[pano9000]

Describe the bug
isDataURI will return false for MIME types, that have an underscore in them
Same issue as #2119

Examples
(bit of a faux example, as the base64 part is not really a "real" application/media_control+xml type file, but that is beyond the point here, as we only care for the MIME type part here)

data:application/media_control+xml;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAIBAMAAAA2IaO4AAAAFVBMVEXk5OTn5+ft7e319fX29vb5+fn///++GUmVAAAALUlEQVQIHWNICnYLZnALTgpmMGYIFWYIZTA2ZFAzTTFlSDFVMwVyQhmAwsYMAKDaBy0axX/iAAAAAElFTkSuQmCC

should return true, but currently returns false

I'll send a PR to fix this in a few moments