Fixing SVG upload to S3

- If you try to upload a SVG file to S3, it is not rendered as an image due to the fact that the MIME type is not defined, so in this case, icon can't be showed and loading doesn't disappear.

![screen shot 2014-11-27 at 18 34 22](https://cloud.githubusercontent.com/assets/132146/5220357/0cc0b1fc-7664-11e4-82be-f784731ff6c8.png)