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.

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

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