By default, workflows will not auto start for attachments sent to a library via email.
To enable this functionality, run the following PowerShell script on your SharePoint server.
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue $spWebService = [Microsoft.SharePoint.Administration.SPWebService]::ContentService $spWebService.DeclarativeWorkflowAutoStartOnEmailEnabled = $true $spWebService.Update()
Next, make sure your workflow is configured to start automatically when an item is created.