A TIFF file must be encoded in either little endian or big endian.
The endianess of a TIFF is defined in its first two bytes, which are 4D4D
for little endian, or 4949
for big endian
With an Hexadecimal Editor (e.g. HxD) change the first two bytes to any other value than the valid ones.
Change the extension of a non-TIFF file to ".tiff".