404 when trying to access file which exists

I'm having trouble while trying to access a file in the browser, it says "404 - file not found" but I know for a fact that the file does exist. I can only access it once and then when I refresh it's gone into 404 mode again. When I delete the file, a different kind of 404 message appears then when the file exists. Does anyone know what I'm doing wrong? Server OS: Windows Server 2008 R2 IIS Version: 7.5 Filename: loop.mp4 I did set the mime-type on the server for the .mp4 extension to video/mp4.