Add magic numbers for WEBP images

This is the diff:

```
 >>22   leshort     =2      \b, stereo
 >>22   leshort     >2      \b, %d channels
 >>24   lelong      >0      %d Hz
+>8     string        WEBP    \b, WEBP image
+!:mime image/webp
 >8 string      CDRA        \b, Corel Draw Picture
 !:mime image/x-coreldraw
 >8 string      AVI\040     \b, AVI
```

Thanks for the great library!