Using USB for transferring files (MTP or PTP) to phone or charging using USB I had my Android phone power discharged fully. On fooling/trying to find the root cause saw the following as options in Android which I wasn't aware of before. This is in developer options though. 


Transferring media files through MTP - This seems to be only one way from phone to PC and not the other way around :(
Transferring images or files through a PTP connection (guess it's a lower bandwidth connection) 


Edit : Update - The target PC is running Debian Testing, not MS-Windows, sorry for not sharing that part. 


Connecting MIDI devices 
Charging. 


What was surprising at least to me was you could choose only of the options and when one is chosen say charging the other options cannot be used. 

Is there possibility of transferring media files through PTP from PC to phone or not anymore ?

Also were these options always part of android say 5.0 or it's a new development in 6.0 marshmallow ?

Permission denied while trying to copy some data from computer on to Samsung Galaxy J5
Its a new development in android 6, mainly the location of these options. The USB configuration of Android 6.0 marshmallow is slightly different than other versions.


  In Android 6.0, the options seems to have disappeared. It has in fact
  moved and you can only access it if you’ve enabled developer options.


The first option is to have a “Charging” only option which can be used to connect to a USB port to charge the device without any notification to the computer. This will be the default option for the device and hence the device will not allow to access the files unless it is explicitly changed.

MTP (Media Transfer Protocol)

When Android uses this protocol, it appears to the computer as a “media device.” A standardized protocol for transferring audio files (and other media) to digital music players using Windows Media Player and similar applications. 

You can transfer media files either way.


  Android can choose the files it presents to you, and hide system files
  so you can’t see or modify them. If you attempt to delete or edit a
  file that can’t be modified, the device will refuse the request and
  you’ll see an error message


PTP Picture Transfer Protocol.

When Android uses this protocol, it appears to the computer as a digital camera.


  MTP is actually based on PTP, but adds more features, or “extensions.”
  PTP works similarly to MTP, and is commonly used by digital cameras.
  Any software program that supports grabbing photos from a digital
  camera will support grabbing photos from an Android phone when you
  select the PTP mode. PTP was designed to be a standard protocol for
  communicating with digital cameras.


It is generally recommended on a modern Android devices,  to use MTP rather than PTP  unless you have software that only supports PTP.

Note: It has been noted that you may need to physically remove the SD card after upgrading to Android 6, and putting it back in before you can view the files on your PC

If you're having issues transferring media in your upgraded OS, try troubleshooting using these options:


Make sure you have installed appropriate drivers


Connect your device to your PC and wait. If it doesn’t show up in file explorer, check to see if Windows is searching for drivers. 



If it is, allow it to search and install any drivers it needs. This can happen even if you have connected your Android device to your PC under the same connection type. Do not disconnect the device and allow Windows to finish installing drivers.


Fix corruption issues on your external storage


Use an sd card adapter and insert in the slot on your PC, right click on the external storage device and go to properties, tools then check for errors, fix if necessary the re insert it in your device and check if problem is resolved.



Credits


How to Access Android 6 Marshmallow Files using USB to Windows
How To Set The Default USB Connection Type In Android 6.0
Android USB Connections Explained: MTP, PTP, and USB Mass Storage