To convert a file (often a photo) to a Base64 format you must install the plugin 'File Utilities'

That plugin will create a new action called 'Convert to Base64'

It works on any file stored on your device.

For example, Camera Block, saves a photo on device and the path to that photo in a variable.

You can use that path to convert the file to a B64 format using the Convert to Base64 action.

Simply create a variable to store that new file.