Upload a file not working

Many customers have told me that my form isn't working because one of the fields is to upload a photo. I had it set on only accepting .jpg and .gif files, but removed that to accept all files. However, an error message still pops up that if I select an image file it doesn't meet the extension requirements. It used to work fine and I hadn't made any changes, so why isn't it accepting uploaded image files now? Please help!

I deleted the old upload field and created a new one and for some reason it's working fine now. I don't know what happened to the original one I created. Hopefully it won't happen again.