If you're like me and use Extra Fields for just about any and everything on your site, then you know there needs to be a "File" extra field type. Which such you would be able to "select" files such as: PDF, MP3, DOCX, MP4, etc from instead of using the textfield types which requires finding the file path and then pasting it into the textfield.
This requires hacking K2, but for me this is a necessary evil. I comment the code and add each hack to a Website Documentation file to archive each hack that I perform.
File: /administrator/components/com_k2/models/extrafield.php
Remove: &type=image
From: index.php?option=com_k2&view=media
&type=image
I welcome any proficient K2 programmer to provide a plugin so we would not need to hack the code to receive this functionality.
Note: I have submitted a request for a "File" extra field type in K2v3
github.com/joomlaworks/k2-v3-dev-build/issues/113 
