Added check for ImageMagick

When uploading files the error "No such file or directory" occurs (Windows 7).  This is because mini_magick requires [ImageMagick](http://www.imagemagick.org/script/index.php) to be installed on the host.  ([Source](https://github.com/minimagick/minimagick#requirements)) 

Is this included on OS X or Linux by default?

In any case I added a check to content_uploader to verify ImageMagick is installed before attempting to re-size an image.  

Also for clarity I added the supported file types to the edit slide form.