Alt-sizes for gifs are sometimes not gifs

[Weebs]

Due to memory requirements on a mobile device, my app always attempts to download the smallest possible size when loading gifs. 

More often than not the smaller sizes listed in the Alt-sizes for photos are actually JPGs and not GIFs. They are have ".gif" extensions with JPG headers and there is no way to tell until the file has been downloaded. One solution would be to limit the alt-sizes to actual gif files or rename the ones that are not gifs to jpegs. It will greatly be appreciated and will save time and bandwidth

thanks