Make dropzone accept mp4, webm, and swf files

to match the rest of the code base

For flash files I've used the file extension because the mime type is browser
specific and I've found at least 2 different variations for .swf:
1. `application/vnd.adobe.flash-movie`: wikipedia
2. `application/vnd.adobe.flash.movie`: mailcap, firefox
some other browser may not even know the .swf extension in the first place

app/views/uploads/new.html.erb

@@ -170,7 +170,7 @@
         createImageThumbnails: false,
         addRemoveLinks: false,
         maxFiles: 1,
-        acceptedFiles: "image/jpeg,image/png,image/gif",
+        acceptedFiles: "image/jpeg,image/png,image/gif,video/mp4,video/webm,.swf",
         previewTemplate: '<div class="dz-preview dz-file-preview"><div class="dz-details"><div class="dz-filename"><span data-dz-name></span></div><div class="dz-size" data-dz-size></div></div><div class="dz-progress"><span class="dz-upload" data-dz-uploadprogress></span></div><div class="dz-error-message"><span data-dz-errormessage></span></div></div>',
         init: function() {
           $(".fallback").hide();