i think there is a java action for that in the community commons module. can you try to take a look there?
CommunityCommons has a Java action Base64DecodeToFile.
Derek,
Depending on the version of Mendix your using you can actually do this without the need for java. If you create a message definition of the image you want to create and a mapping. You can create a json import using a string variable which contains the base 64 code. As long as you match contents to the contents of the file document it'll automatically store this in binary format for you. I use this method all the time now.
[
{
"Name": "selfie.jpg",
"Contents":"/9j/4AAQSkZJRgABAQEASABIAAD/4UXERXhpZgAATU0AKgAAAAgACgEOAAIAAAA2AAAIkgEPAAIAAAASAAAIy"
}
]