Custom IO System doesnt work when Importing from stream

Hi!
I was trying to load some .OBJ files from an IOStream. These obj files have attached materials in a separate file. If I'm loading the .obj files using ImportFileFromStream, I get the following error logs:
```
|Information| - ASSIMP - Info, T10160: Import root directory is './'
|Error| - ASSIMP - Error, T10160: OBJ: Unable to locate material file wall.mtl
```

If I load the file from the disk with ImportFIle()
`|Information| - ASSIMP - Info, T16528: Import root directory is 'D:/Dev/GLAssets\'`

I tried using a creating an IO System and setting it to the importer, but it doesnt even get called.

Start by tracing ImportFileFromStream and ImportFile, then inspect how the configured IO System is attached to the importer. Reproduce the OBJ plus wall.mtl case and check why the custom IO callbacks are not reached. Done means stream imports can resolve the referenced material file through the custom IO system.

Written by the indexing model from the issue text.