animated gifs with different extensions aren't recognized as animated

[bzanchet]

On ./lib/paperclip/thumbnail.rb#102 there's an explicit check to confirm that the image extension is "gif". This is preventing perfectly valid animated gif images (see examples) from being recognized as such. Is there a reason for that? Is there a more reliable way of detecting if a given image is animated?

example animated gifs (no extension, and .gifa ):

1. https://a248.e.akamai.net/assets.github.com/img/3ba24873fade1bf1d82a2265dd1e6073f7a8ec47/687474703a2f2f636c2e6c792f3345324e3345334a3030306b34303051316d31672f6769745f62656172642e676966
2. http://mm.yimg.com/image/resize/*/550x4500/true/false/http%3A%2F%2Fmeme.zenfs.com%2Fu%2F6727d2a336896d1f08300636ea6c6fcf7d414952.gifa

[jyurek]

The reason this was done originally was because some PDFs were causing big problems when more than one page was processed. We could possibly use identify to detect a file's format and the number of images it contains, but that would require more steps in the process. If you're willing to submit a patch to that end, or to extend the names of the formats that will be treated as animated, we'll consider it.