FileStreamDisconnectUploadError on v5.0.0 and v6.0.0.alpha
Hello,

I was reading through several issues and the merged PR #81 but still I resolve my current task: I need multiple file attachment to send the files with the mailgun-js api. I tried 
```js
const currentAttachments = await Promise.all(attachments) // fails

// another approach for loop
const currentAttachments = []
for(const attachment of attachments){
 const t = await attachment
 currentAttachments.push(t)
}
```
As soon I choose more than 2 attachments I receive FileStreamDisconnectUploadError on either of the tryouts above. The major issue is that I need to prepare the readable stream to match the api https://github.com/bojand/mailgun-js#attachments. 
But I never really get to the point to map the attachments because after the second file I receive FileStreamDisconnectUploadError. I was reading through some closed issue and upgraded to v6.0.0.alpha but that didn't solved the issue. How can I handle the process of all attachments to send them all via an external API?
