The Mime Type is not important for the browser. It only sets how it has to interpret the data in the stream it gets.
The setting neccessary to open files in browser is 'Open directly in-brower' under 'Other options'.
For my server, it contains:
*.htm;*.html;*.jpg;*.jpeg;*.gif;*.bmp;*.txt;*.swf;*.gif;*.mp?;*.mov;*.wm?
So you have to make sure the *.gif is somewhere in this value.
The list is separated by semi-colon ;
* stands for any character(any count)
? stands for one character.