Support preview of jpeg and png topics

[thomasnordquist]

Could be nice.

Topics can be identified by magic numbers: http://www.ntfs.com/jpeg-signature-format.htm https://en.wikipedia.org/wiki/List_of_file_signatures

JPEG

Magic number: FF D8 FF Trailer: FF D9

PNG

Magic number: 89 50 4E 47 0D 0A 1A 0A

GIF

Magic number: 47 49 46 38 37 61 Magic number: 47 49 46 38 39 61