Django - detect MIME type

index.html

{% for file in mixed_files %}
<a href=""
   {% if "image" in file|mimetype %}class="fancybox"{% endif %}
   {% if "pdf" in file|mimetype %}rel="external"{% endif %}
>foo</a>
{% endfor %}

templatetags.py

from mimetypes import guess_type

@register.filter
def mimetype(f):
    """
    Returns a string of the mime type for a file object
    """
    return guess_type(f.name)[0]