.dtアクセサは、datetimelikeな値でのみ使用可能です。

...
print("filename:"+df7.loc[i].dt)
...
D:\python34\python.exe D:/python34/program/wx_chat/wxDataSort.py

AttributeError: Can only use .dt accessor with datetimelike values

Process finished with exit code 1

#rename
df7.rename(columns={
'dt':'dt1'}, inplace=True)
# to str
print("filename:"+str(df7.loc[i].dt1))

ワークアラウンド

#rename
df7.rename(columns={
'dt':'dt1'}, inplace=True)
# to str
print("filename:"+str(df7.loc[i].dt1))