At the same time exactly the same code works as expected if I try it out in Python interactive window inside ArcMap:
>>>
... import Tkinter as tkinter
... import tkFileDialog
...
>>> def PickFileToAttach():
... root = tkinter.Tk()
... root.withdraw()
... options = {
... 'parent': root,
... 'title': "Select a file to attach",
... 'multiple': False,
... 'filetypes': [('All files', '*')]
... }
... pathFile = tkFileDialog.askopenfilename(**options)
... root.destroy()
... return pathFile
...
>>> PickFileToAttach()
u'C:/Users/***/Documents/ArcGIS-extensions.doc'
>>> PickFileToAttach()
''
>>> PickFileToAttach()
''
>>>