Appearance: Fix form CSS media rendering

Fix the way the form CSS contained in the media attribute
is rendered. This is now an interator and not a single value.
Replace the current method with a for loop.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_states/forms.py

@@ -178,7 +178,7 @@ class WorkflowInstanceTransitionForm(forms.Form):
 
 
 class WorkflowPreviewForm(forms.Form):
-    preview = forms.CharField(widget=WorkflowImageWidget())
+    preview = forms.IntegerField(widget=WorkflowImageWidget())
 
     def __init__(self, *args, **kwargs):
         instance = kwargs.pop('instance', None)