The file in the screenshot does actually - almost - identify itself as a TIFF: The first two bytes are 49 49 but the following two bytes (55 00) should have been 2A 00 if it was indeed a TIFF file.
What seems to happen, then, is that IrfanView only looks at the first two bytes and assumes it's a TIFF - which is of course both lazy and wrong.

From Wikipedia:
Every TIFF begins with a 2-byte indicator of byte order: "II" for little endian and "MM" for big endian byte ordering. The next 2 bytes represent the number 42, selected "for its deep philosophical significance". The 42-reading depends upon the byte order indicated by the 2-byte indicator.

The easy way to find out if the file has been changed is to install WinMerge and compare it with the original. If the files are identical it will tell you so, otherwise it will show what has been changed.