With reference to the Archive attachments from Mail.app messages hint, we can use OpenSSL to convert a specific Mail attachment from Base64 and back using the following command(s):

To decode from Base64:

openssl base64 -d -in <infile> -out <outfile>

Conversely, to encode to Base64:

openssl base64 -in <infile> -out <outfile>

Where infile refers to the input filename (source) and outfile refers to the output filename (destination). Refer to man enc for more detailed information on using OpenSSL commands.