How to create playlist for all files in folder?
I know this is old, but it's in the top results for "Android create playlist from a folder" so I thought I'd add this:

Playlist Creator 2: https://play.google.com/store/apps/details?id=com.ksdeveloper.playlistcreator.v2

(From what I read, the not so good rating comes from the inhability of the developer to explain how it works, but it's really simple)


Download and open the app. (Duh)
Top right> Use file explorer. 
Navigate to the desired folder and long press it. 
Pick "Add whole folder as playlist".
Press the top right icon to open the playlist, name it, Create Playlist.  


And voilà, you'll see the playlist in the Google music app and any common music player.