Base64 encode a file without uploading it

The misconception is worth attacking directly, because it turns up in real systems: base64 is not encryption and not obfuscation. There is no key, no secret and no work involved in reversing it. A password stored base64 encoded is a stored password with an extra step, and a token that looks scrambled in a config file is a token in plain sight.

What it genuinely does is carry bytes through channels that only accept text. Embedding a small image directly inside a document or a stylesheet, putting a file into a JSON field, moving binary content through a system that would otherwise mangle it. Those are all sensible uses and none of them involve secrecy.

Pick the variant to match the consumer. The standard alphabet includes characters that are unsafe in a URL or a file name, so the URL-safe variant substitutes them; line wrapping exists because some older protocols require lines under a set length. A decoder expecting one alphabet will usually fail on the other, which is a common and confusing error.

Encoding a file is arithmetic, so it runs with the network off, which is worth doing once with something you would not want transmitted. It also makes the size point concrete: watch a file grow by roughly a third and you will think twice before embedding a large image in a document as a data URI.