Tested in testbuild 2,
When I navigate to file, from python panel, string property with sub_type =file_path is set to incorrect destination. This happens only if I navigate outside blend file location. So eg. my blend file is in: C:\Users\pl\Desktop\
I navigate to:
c:\pagefile.sys,
the resulting string prop (sub_type =file_path) is:
'//..\..\..\pagefile.sys' but it should be c:\pagefile.sys
Worked ok in blender 2.76.
Blend file with simple panel script, and string property attached to scene:

• run script,
• navigate to some random file placed outside blend file location (random file needs to be on same drive as blend scene, because otherwise path will be correct- and you wont see bug)
• path will be set incorrectly (someting like '//..\..\..\file.abc')