Invalid file extension
Invalid character in extension
The file extension you supplied is invalid. It must e.g. not contain special characters like ", *, :, <, >, ?, \, /, | or spaces. It must also be less than 20 characters long.
What is a file extension anyway
File extensions are used to help Windows find and launch the correct program when you click on a file. For example, if you click on a .PDF document, Windows will automatically launch a PDF viewer if you have one installed.
Software that will open, convert or fix your files
First of all, you should rename your file so it does not contain invalid characters, and then try opening it with a generic file viewer to inspect its content. In that way you may be able to determine what type of file it really is.
Try a universal file viewer