Change automatically resized image names?

Hello,

I had hacked this in a previous version of wp, but it was overwritten in the update to 3.5 and now I can’t figure it out, despite a few hours looking through docs and forums tonight. I feel like the old hack was in the wp-includes/media.php… But now nothing in there looks right or promising. I guess I’m looking for where the name of the newly created files are set.

What I want, specifically: When I upload a photo to wp, it automatically creates smaller images with file names like “image-200×131.jpg”. The problem is that while I can easily make it so the width is always 200, I don’t know what the height is going to be. Instead of “image-200×131.jpg” I want just “image-200.jpg”.

What I’ve tried: Editing of my theme’s function.php file using add_image_size. It ignored the name string I entered and instead used naming convention as above. Simple Image Size plugin – ditto.

I feel like this must be obvious, but I can’t find it and would appreciate help! Thank you!