Fix WebP image detection

[skidder]

Fixed WebP image detection to prevent misidentification of WAV audio files by checking both the RIFF and WEBP markers in the file header. WAV and WebP use the same container (denoted by RIFF), but have different file formats (denoted by the later bytes WEBP and WAV). Added unit tests to verify correctness.

WebP Container Specification

WAV file header bytes

→ hexdump -C tos-intro-3s.wav | head
00000000  52 49 46 46 8a 13 08 00  57 41 56 45 66 6d 74 20  |RIFF....WAVEfmt |
00000010  10 00 00 00 01 00 02 00  44 ac 00 00 10 b1 02 00  |........D.......|
00000020  04 00 10 00 4c 49 53 54  1a 00 00 00 49 4e 46 4f  |....LIST....INFO|
00000030  49 53 46 54 0e 00 00 00  4c 61 76 66 36 30 2e 31  |ISFT....Lavf60.1|
00000040  36 2e 31 30 30 00 64 61  74 61 44 13 08 00 00 00  |6.100.dataD.....|
00000050  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000740  00 00 00 00 00 00 00 00  00 00 00 00 00 00 0b 00  |................|
00000750  05 00 21 00 0a 00 3b 00  0b 00 5b 00 08 00 81 00  |..!...;...[.....|
00000760  fc ff b1 00 fa ff e2 00  f5 ff 0d 01 da ff 37 01  |..............7.|

WebP file header bytes

→ hexdump -C tears_of_steel_no_icc.webp | head
00000000  52 49 46 46 c4 e5 00 00  57 45 42 50 56 50 38 20  |RIFF....WEBPVP8 |
00000010  b8 e5 00 00 10 68 08 9d  01 2a 80 07 20 03 3e 91  |.....h...*.. .>.|
00000020  44 9b 4a 25 b5 b2 32 a8  b3 ba 42 b0 12 09 67 6e  |D.J%..2...B...gn|
00000030  4c a8 92 a4 db 39 fe 9b  8b e4 68 e4 f2 ba f4 ba  |L....9....h.....|
00000040  17 76 5c ff 7d b6 7f b2  ff a6 e5 b3 d4 3f c8 78  |.v\.}........?.x|
00000050  33 fd 17 85 77 a8 ff 9e  f6 04 f3 06 c5 93 ba de  |3...w...........|
00000060  c1 9e 15 fc ea fd c5 ea  1b ff 77 d2 4b f7 2f fb  |..........w.K./.|
00000070  7d 42 ff ee f3 8d f6 ee  0e a6 26 f8 7a 23 51 9c  |}B........&.z#Q.|
00000080  78 74 7f d4 f3 9d b4 3b  f7 1f fc 3c c7 7a b1 fe  |xt.....;...<.z..|
00000090  2f bd c5 c2 2f e2 1f fb  7d 13 7c 8b fd 2f fe 3e  |/.../...}.|../.>|