As has been discussed in this question, you can override Matlab's default behaviors when dragging files into Matlab's Command window. The key is to override the openXXX function.
When you drag a file into the Matlab workspace window, if it's an image type, that Matlab knows about, it will load the file as an image. If it's image file it doesn't know about, Matlab will load it as a text file. How does one "teach" Matlab to load image formats it doesn't know about, into the workspace window?