One online tool for BASE64 is Motobit Software's BASE64 encoder/decoder.
Simply paste the BASE64 code into the text box, select the decode the data from a Base64 string (base64 decoding) radio button, as well as the export to a binary file, filename: radio button. Name the file for example "mask.png" and click the convert the source data button.
This should result in a download of the mask as a PNG file.
Thank you StarTools for turning my average photos into amazing ones.
Results are free to publish, as long as they are credited "Image acquisition by Jim Misti".
Please note that most datasets will need to be converted to an uncompressed TIFF format.
You can convert everything you see to a format you find convenient. Give it a try!