See https://slack.dev/node-slack-sdk/web-api#upload-a-file for docs on how this method works from a user perspective.

Here is the code from the node-slack-sdk for this method: https://github.com/slackapi/node-slack-sdk/blob/main/packages/web-api/src/file-upload.ts

@seratch has example code for deno-slack-sdk-based apps that use the above approach, more or less, that we could probably lift from: https://github.com/seratch/slack-timesheet/blob/412d9899c839fe24531a097ab0f1f68eb0513d2a/functions/internals/reports.ts#L549-L590