Instead of declaring imageMimeTypes as an object with a `1` value for each field, and then checking on line [318](https://github.com/mozilla/browserid/pull/3713/files#L0R318) with `in`, we have been using an array, then checking whether something is in the array using indexOf:

```
var imageMimeTypes = ['png', 'gif', 'jpg', 'jpeg', 'svg'];
...
(imageMimeTypes.indexOf(dataMatches[2].toLowerCase() > -1) { 
```