Set the Content-Type on uploads

Most browsers seem to set this if you forget, but some don't.

lib/http-api.js

@@ -205,6 +205,9 @@ module.exports.MatrixHttpApi.prototype = {
             url += "&filename=" + encodeURIComponent(file.name);
 
             xhr.open("POST", url);
+            if (file.type) {
+                xhr.setRequestHeader("Content-Type", file.type);
+            }
             xhr.send(file);
         } else {
             var queryParams = {