provide a way to give a default filename to pasted images...

...in the lightbox overlay

![screen shot 2013-06-26 at 9 55 13 pm](https://f.cloud.github.com/assets/362783/711781/55ed02a8-de9a-11e2-8ece-7cbdbff0ca46.png)

Through the `upload.pasted_image_filename` string.