There's no general directory of file type specs, and some of them are not publically documented. JPEG PNG and GIF are, however:
JPEG, file layout is on page 5.
PNG, file structure starts on page 12.
GIF, this one is not as well-written. The "header" section will tell you what the start of the file means, the rest is scattered in the previous sections.
Generally, you can find the specification for a file type by just searching " specification" on Google, and it'll be the first result. Be prepared to do a lot of tedious byte-counting.