Allow user file upload to work

[timja]

Fixes #966

see slackapi/python-slack-sdk#1326 (comment)

and

#966 (comment)

Testing done

node {
  sh "echo hey > blah.txt"
  sh "echo heyhey > blah1.txt"
  
  def slackResponse = slackSend channel: 'U072BG3CH6E', message: 'Hey', sendAsText: true
 
  slackUploadFile filePath: "*.txt", channel: slackResponse.channelId
}

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