The format is normally unchanged. Did you try to add the image suffix .jpeg / .png and try to open the file? Or download a file and open it with your browser. The image should be displayed without a problem.
I cannot add more information. The uploaded image file is renamed to unique name. There is no change in file format. So if you access the stored file the application should be able to read the file. If the application needs a suffix for detection you might need a different application.
You might open a feature request, that Limesurvey adds a filetype suffix to the unique filenames.