NEW: Show loading indicator when attaching files to UploadField

Currently, no indication is shown to the CMS user than anything is actually happening.

Is the attach functionality used anywhere outside a “standard” `UploadField` (e.g. `AssetUploadField`?) I can’t seem to find any usage of it anywhere else.

Attaching:
![screen shot 2014-03-03 at 21 13 54](https://f.cloud.github.com/assets/1655548/2314766/034b0edc-a31b-11e3-9b37-5183f67e4937.png)

Attaching a second file:
![screen shot 2014-03-03 at 21 14 15](https://f.cloud.github.com/assets/1655548/2314777/16384b0e-a31b-11e3-8a7f-ceb4abfd62c6.png)

Attaching via ‘choose another file’:
![screen shot 2014-03-03 at 21 14 44](https://f.cloud.github.com/assets/1655548/2314782/20786ac2-a31b-11e3-995e-e02a98cd249a.png)