How do I convert to RGB file format?

I need to convert files to RGB or RGBA file format for use as modeling textures on an SGI machine, but the options under PhotoShop's "Save As" do not include RGB. I'm not talking about converting to RGB mode or color model; I'm talking about a file extension like TIFF or JPEG. My files are already in the RGB mode and flattened, but I still don't get RGB as a "Save As" option. I'm pretty sure I was able to do this the last time I made textures, two years ago, but I can't figure out how to do it now. I'm wondering if it has to do with my version upgrade. Right now, I've got PhotoShop CS on Windows XP. I think I was running PhotoShop 6 or 7 two years ago. Can anyone shed some light on this? Thanks!

RGB is not (that I know of) a file format. Thus you cannot find it in "save as." (You are mistaken if you think you were able to do this in the past. RGB has never been a PS file format.)

The (SGI) RGB file format is on the CD of Photoshop, you'll need to add it yourself (usually, one do it at install time)
More info at: <http://www.adobe.com/support/techdocs/32216.htm> Yes, Don, it exists!

I stand corrected Pierre.

Thanks for the help! I was able to install the plug-in. The only wrinkle is that it names the extension as .sgi, not .rgb. I assume that they are one and the same (like .jpeg and .jpg), and that I can just rename my file .rgb.