If I am reading this right, you have the model in the Assets folder, and the textures for it in the Textures folder, and those folders are siblings to each other?
That could be the problem. When a model is exported, it will expect to find the textures in the same location relative to the model file as they were relative to the original model file location from where it was exported.
That is to say, if when exported the texture was in the same folder as the source model, then wherever the hgm file gets put, the texture must be in that same place. If the texture was in a Textures folder that was a child to the location of the source model, then there also must be a Texture folder directly below wherever the hgm file gets uploaded to.
Since you have a texture location that does not descend from the location of the source model file, I am not sure how it exports that as a relative location. But if you try having the texture in a location that descends from the location of the source model file when you export, and then maintain that same relative positioning in your uploading, that might work.
TLDR; For each model type, have a folder for it that has all of its relevant files. Either put the textures directly in there, or in a folder within that folder. When uploading, put the hgm file anywhere but then put the textures in the same place relative to the location of that hgm.
