Default [Q] Open With/default app

I have several apps that can open Sony ARW files, but it seems that Android only assigns those apps to open type "image" files. I can't however get Android to work out they are images files. eg. if I try to open one in ES Explorer or HD Explorer it will ask 'open as what kind of file', I tell it 'image' and then select Rawdroid to 'open always' - the trouble with that is Rawdroid becomes the default image handler but Android still doesn't know that ARW is type image and the 'open as type' always pops up.

Is there somewhere you can flag a file type?

The main reason it's a problem is I'm trying to use an IOgear Mediashair and it's poor app doesn't have an 'open as type' so it simply says 'no app installed that can handle', making the device useless for viewing raw camera files.