Ok , after hours and hours of searching trying etc. I still do not have a slightest clue how to solve this problem.

I really want to know from the experts:-))

It seems that when I upload a TIF file to sql server, only the MIME type of this file is not regonized. All other files wich I upload with the same code are and work fine.
I also tried to add the mimetype to IIS 5.0 but that doesn 't work either. The extention I get from a TIF file is application/octet-bytestream and it should be image/tif

so what to do, who knows...