I am trying to register a custom document type in an iPad app, with the hopes that I can be prompted to work with files on an SD card attached to the iPad through the camera connection kit.

Does anyone know if this is currently possible (or even for sure if it's not possible)? I was hoping that the Camera tab in Photos would use a document interaction controller for types it did not understand, but it seems like Photos does not check for alternate handlers at all - I tried also registering support for the jpg type but was not prompted when using photos and loading in JPG images from an SD card.

There is an SD reader app called Zoomit that appears to read many different types from an SD card:

http://currentphotographer.com/zoomit-application-enabled-sd-card-reader-for-the-iphone/

But it seems that involves custom hardware, that differs from the standard SD reader.