Remove-SlackFiles.ps1

$token = '' # Get from: https://api.slack.com/custom-integrations/legacy-tokens
$user = Invoke-RestMethod -Uri "https://slack.com/api/auth.test?token=$token"

# Making sure we're on the right account. Thanks @veegr!
$user | Select-Object Url, Team, User | format-list
Write-Output "Is this the correct account?"
$choice = Read-Host "[Y/N]" 
Switch ($choice) { 
  Y {Write-Output "`nDeleting with great vengeance and furious anger`n"} 
  N {
    Write-Output "Exiting."
    exit
  } 
  default {
    Write-Warning "Somehow you managed to enter something besides Y or N"
    exit
  }
} 

$userId = $user.user_id
$time = 1508396400  # https://www.wolframalpha.com/input/?i=15+days+ago+in+epoch
$files = Invoke-RestMethod -Uri "https://slack.com/api/files.list?token=$token&user=$userId&count=999&ts_to=$time"

forEach ($file in $files.files) {
  $fileId = $file.id
  Write-Output "[*] Deleting $fileId"
  $result = Invoke-RestMethod -Uri "https://slack.com/api/files.delete?token=$token&file=$fileId"
  if ($result.ok) {
    Write-Output "[*] Deleted $fileId"
  }
  else {
    Write-Output "[!] Error: $($result.error)"
  }
}