Why is this a problem?

Having the extra flexibility is a benefit to the user interface, because
you can end up in situations where files happen not to be named
correctly.  That is why you shouldn't remove the "All Files" filter
option.  I think it is a mistake to assume that you have complete
control of the environment in which your application can run, or that
the software is smarter than the user.

By all means, add the image file filter and use that as the default
setting, but leave the option to select all files in as well.  You will
have a more robust user interface, and your users will be thankful.

I don't really see any advantage to disabling the all files option,
since you would still need to handle mal-formed or mis-named files when
you try to load image files anyway.  So if you are worried about someone
deliberately changing the filter and choosing a non-image file, you
would already have the code in place to trap the problem and complain.
You need that code anyway, so it isn't any more work to make the
interface flexible.

Thanks a lot!