fix: properly decode base64 strings inside read

[Rich-Harris]

kind of incredible that atob and btoa, aside from the spectacularly shite naming, straight up don't work. clown shoes ass platform.

anyway, this fixes the weird characters identified in #11679. thanks @danielzsh and @tmarsik42 for the invaluable sleuthing here, this could have taken a while to figure out

ugh i guess i should add a test

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