Saving multible files in .png format

I'm currently working on a claymation in which I'm using photoshop to extract backgrounds. each shot I'm saving as a .png for import to flash for editing. When I first started,(and after I had already saved a couple) each time I went to Save As and typed in the name I wanted, .png was already selected in the file type box. now I don't know what button I hit by mistake, but for some reason it's defaulting to the .psd everytime and I have to scroll down to get the right file extension. with over 1000 shots to edit it would save me alot of time if I could get it back to remembering that I want to save as .png for whatever session I'm working. Any suggestions?

Thank you in advance
Brian