Documentation/comments added in file.py on how to return ObjectId of already existed file

**Returning *ObjectId* from ```save_file()```**
- If file already exist and ```save_file()``` of ```file.py``` is called then : 
  - Previously it was returning only file name.
  - But now by supplying one more argument of ```oid=True``` to ```save_file()```, we can get ObjectId of file
  - e.g : ```obs_image = save_file(each,title,userid,group_id, content_org, tags, img_type, language, usrname, access_policy, **oid=True**)```
  - If no extra argument of ```**oid=True**``` supplied to ```save_file()``` it will return file name as it was. (default/previous behavior is preserved)


**Checking MIME types of files in Observation App** : 
- When we upload file in marker popup : 
   - It's available MIME type will be shown.
   - But some corrupt files does not have mime type then text - ```file``` will be shown