Slack logs - hide slugs for thumbnails

A little noisy in the slack thread when thumbnails are shown.  Just show the main slug.

```
CA-Drought-Update-10-25-21-pdf-116x150,
CA-Drought-Update-10-25-21-pdf-232x300,
CA-Drought-Update-10-25-21-pdf-386x500,
CA-Drought-Update-10-25-21-pdf,
CA-Drought-Update-10-25-21
```

...should just be...
`CA-Drought-Update-10-25-21`

continue to list all the files in the replies.