Specify image extension equivalents in config

[entonio]

A number of raw data images bear the extension of the machine that uses them. For instance, the 720 KB images of the Korg 01/W use .01w. In order to use them with FF, their extension has to be changed to .img. That's not practical in terms of archiving, because in your computer floppy images can have many provenances and it's ideal if each image keeps the original extension. It would be great if 01w could be specified as a synonym of img in the config.