In Luna MTP devices were prefixed with "gphoto2://" and were properly thumbnailed; Shotwell could also open files from it. Or at least, that was the case for my Sailfish-powered phone.
In Freya, MTP paths look like "mtp://[usb:008,003]/Phone%20Memory/something.txt" and there are no thumbnails. Files also cannot be opened, neither in theoretically gvfs-supporting programs like Scratch nor via programs that only accept loca file names. They receive the path "/run/user/1000/gvfs/mtp:host=%5Busb%3A008%2C003%5D/Phone Memory/something.txt" but that doesn't seem to work either.
Files indicates that the size of any file on MTP device "0 bytes". It also cannot create new files via context menu entry, but it can create new folders. It's also able to copy files to and from MTP devices.
This does look a bit like a problem with GVFS in general (i.e. its MTP backend being lousy) and not with Files in particular, so I'm reporting it both against Files and the OS.
According to Wikipedia under a [citation needed], MTP requires copying the file to the host before opening it. So that would explain part of the weirdness.