have noticed that some custom icons and also some custom models, does not work in the editor but, they do work ingame. that is because the editor often requires mdl files for models while the game can make do with mdx files. a simple solution would be to make the editor require only mdx files.
I do not think that is the problem since they likely use the same code to load and display models.
More likely a caching problem.
some times, custom icons never work in the editor but always ingame, no matter how many times i shut down and restart the editor.
if you would suddenly change it to reguire only mdx files you would propably make tousands and tousands of models unusamble. is that what you want?
all models these days is using mdx files, no models i have found is using mdl files.
It is likely that during alpha development Warcraft III used .mdl which is why World Editor still references the format as such. This is a textual format which until recent obfuscated versions of Warcraft III the game still contained a parser for in the executable. If one could get .mdl to load in older versions I am not sure, between improper convert implementations and World Editor getting confused it seemed impossible so maybe it was dead code. The fact converters for this format exist is strange, meaning it might have been used by an alpha, beta or maybe art tools.
As far as I can remember Warcraft III uses .mdx which is a binary model format. World Editor also uses .mdx however it does incorrectly rename model path strings to .mdl in some views despite it still resulting in the correct model path being resolved.