How to decode base64-encoded inline attachments in email files saved from Thunderbird or Gmail?

Although base64 [--decode] works well when encoding and decoding files locally, I can't decode a base64-encoded inline attachment in an eml file saved from Thunderbird or Gmail. I observed that base64-encoded blocks in all eml files are similar to what base64 generates, but lines are of different length: the lines in base64-encoded blocks of text in eml files consist of **72 characters**, while the locally generated base64-encoded files consist of **76-character-long** lines. I suspect this discrepancy is why I cannot successfully use base64 --decode on snippets of eml files. Is my hypothesis true? How can I decode these attachments?