In the last paragraph of your post - what is the file extension on the file being opened?
In other words, what type of image do 'you' think it is, compared to the image type that DPP believes it to be (although it seems to be an image type the DPP cannot 'understand' and hence cannot open), compared to the image type that your Operating System may or may not be able to understand, compared to the image type that PixInsight may (indeed, should) be able to open?
The 'devil is in the detail' here - that file extension is a crucial piece of information, and you really should have the display of all file extensions 'enabled' within your OS, and should really be quoting the extension in the file name whenever you refer to any file. A filename without a file extension is not really a filename at all.