Resourcetype servlet resolution

Hi All,

As "AssetDownloadServlet" is registered with following properties:-

@Properties({@Property(
  name = "sling.servlet.resourceTypes",
   value = {"sling/servlet/default"}
), @Property(
  name = "sling.servlet.methods",
   value = {"GET", "POST"}
), @Property(
  name = "sling.servlet.selectors",
   value = {"assetdownload"}
)})

then how this servlet get invoked from a ajax call having url pathtoaasset.assetdownload.zip/suffix? since an aem asset doesn't have a resourcetype property either?

can someone provide me more details about this?