Ok,
i had tried making a new folder and adding it there with the same result, (had to test for the heck of it)

since you mentioned file permissions, i decided to take a look at the File table in the database, and this record has some differences from the others, so it might just be file permissions as you mentioned.

Note: I'm even getting a 403 Forbidden on images that I am importing.

OK, but this is a different issue alltogether...

1. by 'rights' I mean the webserver must have read/write permission in the assets/ directory...

2. I don't know what platform your site is on. (WAMP, LAMP, IIS?) but you might check the .htaccess file in your /assets directory, that denies certain filetypes. There have been issues with that before, though I can't find the post at the moment/nor the solution. Just temporary remove the file and see what gives.

If this works, search the forums, 'cause I know there's a solution hiding here somewhere :-)

btw: don't send tiff images, better to create a jpg/png/gif for it :-)

Yeah, removing the .htaccess file did the trick

Any idea how i should fix the file for it to work properly? :S

Nevermind, thanks, i opened the file and added the unity3d extension to the regular expression of allowed extensions and it works.

:D thanks for all your help!