Okay, so there is something else going on. Since I cant look over your shoulder and see what the actual names and locations are, here is what I suggest:
1 - Edit in Explorer -> An explorer will open showing you the content of your asset
2 - Make sure the thumb is in the same folder as the config file; not in a subfolder
3 - Rename the thumb to "thumb.jpg"; no capital letters and especially no spaces in the name.
4 - Open the config.txt file in notepad; I prefer my editors to have no brain of their own when I try to solve a bug.
5 - Make sure your config.txt has only 1 "thumbnails" section in it and that it looks like this:
Code:
thumbnails
{
0
{
image "thumb.jpg"
width 240
height 180
}
}
I know, it looks silly, but the more basic and easy it looks, the less risk of bugs / mistakes / errors.
It would not be the first time I have spend hours trying to find a bug and then discover it is a silly space or capital letter. The difference between a / and a \ also has cost me to many minutes of my life. I would not be surprices if you are in the same sort of situation.
Another point is, will it be rejected if I upload it without a thumbnail?
I never tried it without, so I cant help you there. But since the thumb and name are the main visible items in the asset list, I think your work should have a thumb to get the attention it deserves.
Edit:
If you have been editing the config.txt file in a different editor then notepad or CCP, then maybe that editor decided to "help" you with adding spaces, quotes, tabs or whatever other junk todays editors can think of by themselves. In that situation: Do a brainless count of all the " and check if you got an even number. Same goes for { } ... see if for every { you got a } . I know, not the most fun thing todo, but it does explain some errors every now and then.