When reporting an error like this, it helps to provide a media ID that was provided when you attempted the upload.
In this case, the GIF image is corrupt. I used the identify tool from ImageMagick to check this, and it contains three weird frames that do not match the rest of the format:
$ identify ~/Downloads/badmedia.gif git:(master|✚1
/Users/apiper/Downloads/badmedia.gif[0] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.000u 0:00.000
/Users/apiper/Downloads/badmedia.gif[1] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.000u 0:00.000
/Users/apiper/Downloads/badmedia.gif[2] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.000u 0:00.000
/Users/apiper/Downloads/badmedia.gif[3] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[4] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[5] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[6] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[7] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[8] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[9] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[10] GIF 162x272 162x272+0+0 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[11] GIF 11530x29204 162x272+25010+61751 8-bit sRGB 8c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[12] GIF 4867x19749 162x272+774+1109 8-bit sRGB 256c 350007B 0.010u 0:00.000
/Users/apiper/Downloads/badmedia.gif[13] GIF 536x45930 162x272+49163+7102 8-bit sRGB 2c 350007B 0.010u 0:00.000
identify: corrupt image `/Users/apiper/Downloads/badmedia.gif' @ error/gif.c/PingGIFImage/940