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.
This is a basic workflow showing how real-world, imperfect data from a DSLR can be processed in StarTools.
StarTools is a new type of image processing application for astrophotography that tracks noise propagation as you process.
You can convert everything you see to a format you find convenient. Give it a try!