PNG's sent via email or to Slack are not showing - URL looks very strange

I upgraded to version 6.2.4 and now noticing a strange issue. I have Grafana configured to include images when alerting for both email and Slack. If I send a test email the PNG shows up just fine.
test
However, when an actual alert is triggered it never shows the PNG in Slack now and the email version is blank.

Within my configuration I have set the following:
#################################### External Image Storage ##############
[external_image_storage]
# You can choose between (s3, webdav, gcs, azure_blob, local)
provider = local

In terms of rendering, I have not set anything…
[rendering]
# Options to configure external image rendering server like https://github.com/grafana/grafana-image-renderer
;server_url =
;callback_url =

However, rending has been set this way since day 1 and it worked before v6.2.4 so I am not sure that is the cause.

I just was watching the log when one of the alerts fired.
t=2019-06-24T09:51:04-0400 lvl=info msg=Rendering logger=rendering path=“d-solo/AnOu_fdik/tibco-ems-error-queue-monitoring-testing?orgId=1&panelId=18”
t=2019-06-24T09:51:06-0400 lvl=info msg=“uploaded screenshot of alert to external image store” logger=alerting.notifier url=http://prometheus.xxxxxxxxx.com:xxxx/public/img/attachments/1L4PF6LQV9T58dH8r3qw.png
t=2019-06-24T09:51:06-0400 lvl=info msg=“Executing slack notification” logger=alerting.notifier.slack ruleId=3 notification=“xxxxxxonServicxxxxxx”
t=2019-06-24T09:51:06-0400 lvl=info msg=“Sending alert notification to” logger=alerting.notifier.email addresses=[xxxxxxxxxx@xxxxxxxxxx.com]

I checked the C:\Grafana\public\img\attachments directory and it is empty yet it says it created the image file there.

Any ideas what would be causing this? It worked just fine before the upgrade.

Thanks!