Fixed issue where uploaded attachments weren't being encoded and deco…

[tonyanziano]

…ded properly.

Fixes #1666

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Also made sure that this did not undo any of the work done in #1661

Tested against the attachment sample -- sample 15 -- in BotBuilder-Samples, as well as against the scenario provided by the user in #1666.

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The sample JSON file I uploaded through the Emulator:

{
  "payload": "hey :)"
}

My bot code:

onTurn() {
  return async (context) => {

    if (context.activity.attachments && context.activity.attachments.length && context.activity.attachments[0]) {
      const activity = context.activity.attachments[0];

      // json payload 
      if (activity.contentType === 'application/json') {
        const attachmentContentUrl = activity.contentUrl;
        const response = await fetch(attachmentContentUrl);
        const attachmentData = await response.json(); // { type: buffer, data: <array of bytes> }
        const readableJsonString = Buffer.from(attachmentData.data).toString();
        await context.sendActivity(`Decoded JSON payload: ${readableJsonString}`);
      }
    }
}

Running this in the Emulator: