that is weird because once opened in Gimp a file is not anymore a jpg ,bmp, png or whatever but a gimp-imagine that may be in RGB, RGBA, index or Greyscale mode.
So the original format ,jpg,xcf,bmp does not matter just need to be a kind of file that gimp may open
"Can someone help me? "
Try again checking that the image is in RGB mode
gimp-histogram
You should decribe what