Your upload file should be properly prepared. Firstly, name your files in a proper way. All of your uploaded files (3D file, archive file, folders, textures) should be name without use of white spaces and without use of the any national characters, like ś, ć, ą, ę, ó, ź, ż, ü, ß, etc.
Then, check whether your 3D program generated only the 3D file or also additional files (e.g., a folder with textures) during export. In case more than one file was created during the export, remember to pack the whole package into one of the supported archive files (see Supported file formats). Please don’t change the output (hierarchy, names, extensions etc.) of the exported 3D models and/or textures, materials. Pack the exported file package without making any changes to the archive file, and don't create a parent folder in the archive file! Remember also to name your archive file exactly the same as you 3D file inside.