I'm getting an error here when I try to upload a ZIP file that contains non-image files. For example, on my MAC I zipped up a directory that I thought had only images, but the ZIP also contained files like this:

```sh
.DS_Store
__MACOSX/
```

Here's the specific error I'm getting:

```sh
Internal Server Error: /image-repo/upload
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Users/abarrett/Development/harvardcards/harvardcards/apps/flash/views/img_repo.py", line 28, in img_upload
    all_imgs = services.handle_uploaded_img_repo(imgs_file)
  File "/Users/abarrett/Development/harvardcards/harvardcards/apps/flash/services.py", line 155, in handle_uploaded_img_repo
    img = Image.open(dataEnc)
  File "/usr/local/lib/python2.7/site-packages/PIL/Image.py", line 2006, in open
    raise IOError("cannot identify image file")
IOError: cannot identify image file
[28/May/2014 11:36:41] "POST /image-repo/upload HTTP/1.1" 500 74040
```