Textures don't load with .gltf ?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

Apparently Open3D has support for textures in .gltf (https://github.com/isl-org/Open3D/issues/1775#issuecomment-720813575) but when I render a 3D model it appears without texture and I see the following warning in the console.

```
FEngine (64 bits) created at 0x7f88fe000000 (threading is enabled)
FEngine resolved backend: OpenGL
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
[Open3D WARNING] Read geometry::Image failed: missing file extension.
```

### Steps to reproduce the bug

```python
import cv2
import glob
import numpy as np
import open3d as o3d

render = o3d.visualization.rendering.OffscreenRenderer(width=640, height=640)

# https://www.dropbox.com/s/b4ovqdaxvuwhkyj/house.gltf.zip?dl=0
mesh = o3d.io.read_triangle_mesh(f'house/model.gltf')
material = o3d.visualization.rendering.MaterialRecord() # Create material
render.scene.add_geometry(f'mesh', mesh, material)

cam = o3d.camera.PinholeCameraParameters()
cam.extrinsic = np.asarray([[1,0,0,0],[0,1,0,0],[0,0,1,150],[0,0,0,1]]) # Translate infront of camera by 5 unit
cam.intrinsic = o3d.camera.PinholeCameraIntrinsic(640, 640, 640, 640, 320, 320)
render.setup_camera(cam.intrinsic, cam.extrinsic)

# Render image
image = np.asarray(render.render_to_image())
image = cv2.cvtColor(image, cv2.COLOR_RGB2BGR) # Convert Open3D RGB to OpenCV BGR
cv2.imshow('image', image)
cv2.waitKey(0)
```

### Error message

### Expected behavior

Textured

mesh.

### Open3D, Python and System information

```markdown
- Operating system: macOS 10.15
- Python version: Python 3.7.10
- Open3D version: 0.15.2+b48fef1
- System architecture: arm64
- Is this a remote workstation?: no
- How did you install Open3D?: pip
- Compiler version clang 10.0.0
```

### Additional information

_No response_