The file is corrupt or has an incorrect extension. Please check the file and upload again.
How does it know this? Could the program be wrong?
I got that message once. I was trying to upload a .jpeg file (Files need to be in JPG, GIF, PNG, or PDF format).
I opened the file using "Paint" and saved it as a PNG file, after doing that I could upload the image.
There's this other post. Do you remember what the solution was?