That error message is referring to the setting in the web.config file with a key of "Graffiti:FileBrowser:Editable". That setting only allows ".view .txt .css .js .xml theme.config" by default.
What's going on is that the edit theme page (aka Personalize) is intended for working with editable text files. It only shows files that match the editable extension list, and won't let you upload any other file types. There's another page where you can
view ALL files and upload any type of file - the File Browser. You can get to it by going to Site Options > Utilities > File Browser.
We should make this simpler to figure out.. and perhaps not bury the File Browser so deeply in the navigation. Sorry for the confusion. I'm thinking for v2.0 we should spend some time reorganizing the admin navigation.