Confusion with Gravatar URLs.

Question

I am having a bit of trouble getting Gravatars to work properly:

When I request the following:

http://gravatar.com/avatar/8a17d0d0d8bdf6a8d527bbc943a17cf8.jpg?s=64&d=identicon

Firefox proudly displays the following:

...indicating that the file is a PNG image.

This confuses me - I thought Gravatars were JPEG images. It seems like they can be either. How can I find out if a given image is PNG or JPEG preferably without downloading it first?

---

Note: Some people are reporting that Gravatar only returns PNG images. Please explain this:

http://gravatar.com/avatar/03cd042b82ac85b2c5fe0757a94e0413?s=64&d=identicon

Answer 1

The end tag is JPG, but the string is interpreted by the server and the corresponding image is sent. In this case a PNG.

Answer 2

If Gravatar icons have accurate MIME types assigned by the server you're accessing them from, just check that. It should be image/jpeg for JPEGs and image/png for PNGs.

Failing that...

http://en.wikipedia.org/wiki/Portable_Network_Graphics#File_header

A PNG file starts with an 8-byte signature. The hexadecimal byte values are 89 50 4E 47 0D 0A 1A 0A; the decimal values are 137 80 78 71 13 10 26 10.

So just check the eight bytes at the beginning of the file; if it's a PNG, it'll have the stated values in those bytes, and if not, it won't. Just download the file, possibly store it somewhere temporarily (which shouldn't be too hard considering it shouldn't be too big), and process it differently depending on what the header contains. You can always change the file extension and then use PHP's graphics library if you saved it as the wrong type at first. (Or are you not allowed to do that?)

As a side note, my favorite bit about the PNG header:

50 4E 47 In ASCII, the letters PNG, allowing a person to identify the format easily if it is viewed in a text editor.