Allow any asset be be added as layer

[philvarner]

See #198 for similar issue (likely a bug) for GeoJSON.

I have a KML asset in an item that I'd like to add as a layer, but the "add" button is disabled because it's not on the short, hard-coded list of media types. QGIS supports many more types than just this, and they should be allowed to be added. There are also many plugins that support other types. Also, there are several variations of these media type strings that should be allowed also.

One possibility is to always have an "Add asset as layer" button, but make display a warning if it's not one of the most common types.