a potential resolution to tootsuite/mastodon#3804, achieved by not
encouraging people to use upload methods without filenames
my rationale for this being the right solution is that it brings it to
parity with the media upload method, which has already proven to be
enough for library developers to understand. it should be enough that,
early in the docs, it says that 'When a file parameter is mentioned, a
form-encoded upload is expected.'
i wouldn't mind so much if the base64 upload method didn't expose such
an insidious bug, but even if it didn't, it seems reasonable to want to
encourage developers to upload all media in a consistent way
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