Slack files.upload deprecation

[jonnilundy]

Hey @isaaci, I believe i found a bug in the new files.completeUploadExternal method.

Slack `files.completeUploadExternal` method uses string instead of object for `files` property

My goal: I'm building a daily reporting workflow and I want to create a graph using chart.js and then upload that to Slack. Issue: On the last step of the flow, I need to hit the files.completeUploadExternal slack method and pass an object on the files` property. Unfortunately, the property is set to be a string and I don't see a way to override it. This results in a failed step and no way to finish uploading a file to Slack. Steps I've taken to troubleshoot: I've tried changing the format to a…