If you pass `revision_mime` without `content_resourceSubType=file`, you get an error:

`$.post('/api/content/reprocessPreviews', {'revision_mime': 'application/pdf'});`

I wind up with this error in the logs:

```
[2013-09-19T18:15:14.093Z] ERROR: oae/22542 on branden-macbook.local: An uncaught exception was raised to the application.
    TypeError: Object function (obj) {
        if (obj instanceof _) return obj;
        if (!(this instanceof _)) return new _(obj);
        this._wrapped = obj;
      } has no method 'indexBy'
        at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-preview-processor/lib/api.js:541:45
        at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-content/lib/internal/dao.revisions.js:141:13
        at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-content/lib/internal/dao.revisions.js:97:9
        at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-util/lib/cassandra.js:745:9
        at onReturn (/Users/branden/Source/oaeproject/Hilary/node_modules/helenus/lib/connection.js:444:7)
        at onReturn (/Users/branden/Source/oaeproject/Hilary/node_modules/helenus/lib/connection.js:379:7)
        at client._reqs.(anonymous function) (/Users/branden/Source/oaeproject/Hilary/node_modules/helenus/node_modules/helenus-thrift/lib/thrift/connection.js:80:11)
        at Object.CassandraClient.recv_execute_cql_query (/Users/branden/Source/oaeproject/Hilary/node_modules/helenus/lib/cassandra/Cassandra.js:8295:12)
        at /Users/branden/Source/oaeproject/Hilary/node_modules/helenus/node_modules/helenus-thrift/lib/thrift/connection.js:83:37
        at Socket.<anonymous> (/Users/branden/Source/oaeproject/Hilary/node_modules/helenus/node_modules/helenus-thrift/lib/thrift/transport.js:70:9)
```

If the state is "Undefined", there should be validation against it.