Event material should be <attachment>, not <url>

We have recently introduced events, which can contain links to extra materials (such as the handbook) and talks, which can link to videos, for example:
https://github.com/acl-org/acl-anthology/blob/c7127b85f4f6979410f05424b0d6a3785ef3af38/data/xml/2022.acl.xml#L12299-L12322

These all use a `` tag, which got an extra "type" attribute just for this purpose, for example:

```xml
2022.acl.keynote1.mp4
```

I propose to remove this "type" attribute from `` again and just change this to be an ``. Attachments already support "type" attributes, and what's more, they are actually intended for local filenames, contrary to `` which up to this change was exclusively for fully-qualified URLs. Additionally, this would mean adding a checksum for these files, which seems like a good idea.