refactor: file

[sahil28297]

Asana Link

Fixes:

• Issue where if same file was attached to two different documents and if one was changed from private to public (or vice-versa), the other one did not change which might cause security issues.
  
• Fixes attached_to_field null values.
• Throws error if get_content was called for a folder.
• Throws error if file_name contains separators which used to break while storing the file in disk ( file system).
• Appends files of Home/Attachments folder as well in get_files_in_folder function.