Bug resolved in collection module(image & video)

Previously heterogeneous and homogeneous collection for images and videos are not working
Now bug fixed , little modification in querry 
Modification in : 
```
file.py  --> rename 'mime' to 'mime_type' 
methods.py   -->  get_darwers() function modified for image & videos according to their member_of field and for heterogeneous section type_of field is removed from querry 
```