Support Blob and File as value types for backgroundImage and environmentImage

Currently, we detect HDR images by their file extension. This is great if you only ever use URLs to specify the image, but it falls apart for drag/drop (such as with Ricardo's demo [here](https://truthful-roof.glitch.me/)) and other workflows where the file extension is obscured by [`URL.createObjectURL`](https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL).

There are a few ways to address this that might be worth discussing. However, I would recommend that for the user's benefit we should transparently support `Blob` and `File` as valid values for `backgroundImage` and `environmentImage`. Both of these values should give us enough information to detect an HDR image (mime-type and filename, respectively). This will allow users to directly assign these properties and not worry about the finer details required to accomplish what they are trying to do.