I'm a new user and had this same problem with opening OBJ files with no textures. Luckily I have some programming experience and started digging into the MTL file that is generated with the OBJ file. This is where my problem was at.
Basically, that MTL file links the model to it's textures in the code. That said, if creator Joe Schmoe exported an OBJ, the corresponding MTL file is going to link the image files as listed on HIS PERSONAL COMPUTER wherever that file may be. If you open the MTL file in any text editor, you'll find it's easy to navigate. Obviously you'll want to just change the file path that it points to. See below:
Tf 1.0000 1.0000 1.0000
Ka 0.4471 0.4471 0.4471
Kd 0.4471 0.4471 0.4471
Ks 0.5580 0.5580 0.5580
Ke 0.0000 0.0000 0.0000
You simpy need to change this to your new file path, or just leave a space between code and image:
* In this example, the OBJ and JPG would be right next to each other in the same folder.
REFERENCE (educate yourself ):
Read this to get a deep understanding of how the MTL file works.
I'm still having trouble with 3DS and FBX files but don't think it's an error on Fluid Importer side. As told to me by TurboSquid directly, there are many variations and output methods to all 3D file formats. Will usually take some work to get it all working properly. The usual pain in the A$$ that we all go through with technical work like ours.
Hope this helps you and anyone else that comes along this post!
Maybe Fluid Interactive guys can add some info like this to the site