Support fb send upload

[siygle]

Solved #233

Use filedata to assign upload file/image describe on Content Types page in FB messenger doc. It will do a basic file validate (fs.existsSync) then stream the file using form-data.

e.g., upload png file

bot.reply(message, {
  attachment: {
    type: 'image',
    payload: {}
  },
  filedata: UPLOAD_FILE_PATH
})

[jonchurch]

Haven't used the upload api myself, but the docs say the url for uploads is:
v2.6/me/message_attachments
Ahh, misunderstood the purpose of this PR! Carry on 😉

[ouadie-lahdioui]

@jonchurch you made me think to bring the functionality that you misunderstood to Botkit 😈

If you don't already start to work on 😬

[ouadie-lahdioui]

Hello champion,

Thanks for this awesome work 👍

[ouadie-lahdioui]

What do you think about initializing requestObject with POST method and uri to avoid duplication ?

var requestObj = {
      method: 'POST',
      uri: 'https://' + api_host + '/v2.6/me/messages'
};

[ouadie-lahdioui]

If you initialised the requestObj with the http verbe POST and the uri, you just need to set here the formData :

requestObj.formData = prepareFormdata;

[ouadie-lahdioui]

@jonchurch you made me think to bring the functionality that you misunderstood to Botkit 😈

If you don't already start to work on 😬

[ouadie-lahdioui]

Your code is really awesome, but what do you think about starting with checking for filedata to avoid a slightly duplication ?

if (key === 'filedata') {
   if (!fs.existsSync(facebook_message[key])) return cb(new Error('filedata not exist'));
       prepareFormdata[key] = fs.createReadStream(facebook_message[key]);
   } else if (typeof facebook_message[key] === 'object') {
       prepareFormdata[key] = JSON.stringify(facebook_message[key]);
   } else {
       prepareFormdata[key] = facebook_message[key];
   }

[stale]

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