This specific proposal will only really help with the last part - it will let you write

    return `data:application/octet-stream;base64,${(value as Uint8Array).toBase64()}`;

but you'll still have to do the work of reading the stream to a buffer yourself.

There is a _very_ early stage (as in, it's literally just an idea one person had, which may never happen) proposal [1] to do zero-copy ArrayBuffer concatenation, which would further simplify this - once you'd collected the chunks you could `value = new Uint8Array(ArrayBuffer.of(chunks.map(chunk => chunk.buffer))` instead of manual concatenation.

Finally, there's the Array.fromAsync proposal [2] and/or async iterator helpers proposal [3] (which I am also working on), which would make it easier to collect the chunks. Putting these together, you'd get something like

    if (isReadableStream<Uint8Array>(value)) {
      const chunks = await Array.fromAsync(value);

      if (chunks[0].byteLength) {
        value = new Uint8Array(ArrayBuffer.of(...chunks.map(chunk => chunk.buffer)));
      } else {
        throw new Error(`Unrecognized readable stream type: ${ chunks[0].constructor.name }`);
      }
    }

    return  `data:application/octet-stream;base64,${(value as Uint8Array).toBase64()}`;
    

[1] https://github.com/jasnell/proposal-zero-copy-arraybuffer-li...

[2] https://github.com/tc39/proposal-array-from-async

[3] https://github.com/tc39/proposal-async-iterator-helpers