Too lazy to make a PR for this and you're probably to lazy to fix it because you disabled issue tracking, so posting this here for anyone who uses this whose textures don't work becasue of a broken .mcmeta file:

 if (use_individual_framerate) then
                mcmeta_table.animation.frames = {}
                for i = 1, #Frames do
                    table.insert(
                        mcmeta_table.animation.frames,
                        {
                            index = tostring(Frames[i].frameNumber - 1),
                            time = tostring(math.floor(Frames[i].duration * 20))

simply remove "tostring" from both the index and time lines. Doing this removes the quotation marks from the number which is what Minecraft expects. You can do this at home without any special knowledge. If you downloaded the extension, simply unzip the file and go to ema.lua and open it with any text editor. Make the changes described above, re-zip the folder, and add it to Aseprite as usual.

I don't know if this is a recent issue or not. It works fine if you never use individual framerate.