When you upload a model that require a "custom" texture, HiveWorkshop ask you to upload the texture alongside the mode, it is linked with the "Egg attachment" resource, as you can see it weight 11.57KB, that's the size of the model + the texture (64x64 icon in this case). When you download the resource, you'll find the texture & the model in the same folder.
Here's where I found the icon used as the egg texture ->
File:Inv egg 03.png