Sorry, didn't realise it was Windows 7.
Have to admit, I'm no Windows 7 expert. Could you write this in a text document, save it as "import.reg" and double click it to add it to the registry? It works in XP.
Just replace "YOUREXTENSION" with the extension you're trying to add?
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.YOUREXTENSION]
"PerceivedType"="image"
@="jpegfile"
"Content Type"="image/jpeg"
[HKEY_CLASSES_ROOT\.YOUREXTENSION\OpenWithProgids]
"jpegfile"=hex(0):
[HKEY_CLASSES_ROOT\.YOUREXTENSION\PersistentHandler]
@="{098f2470-bae0-11cd-b579-08002b30bfeb}"