Plugin Installation Troubles & Question

[MikuruX]

I'm having a issue with trying to install any kind of plugin into my site. I've tried various ones and everything goes okay until I hit the install button. The installation will drag out for a minute or two before it kicks me out of the admin area and back to the admin login screen. When I try to log back in, it sends me back plugin section where it shows that the plugin is still trying to install but moments later it will kick me right back to the admin login screen. This will repeat infinitely until I close out the tab that I'm working on for the page in my browser. When I open a new tab and go back to the admin controls plugin section, I find multiple incomplete copies of the installation in my list and all I can do is delete them. Has anyone else ran into this problem?

 

Also, the question I wanted to ask was how do I go about adding additional file extensions to the list of files I can upload into the gallery? I'm trying to add the ability upload ".swf" files into my gallery but it only takes ".flv" files.