I have an automation triggered by File Drop. when the file is dropped , it will be imported to a staging data extention(overwrite). now here is an odd thing: when I upload a file name as "AAA_TEST_%%Year%%%%Month%%%%Day%%_%%Hour%%.csv" , it can trigger the automation, and automation runs successfully. However, the data in the staging DE still can't be overwrite and still previous data ,seems the import activity doesn't work. I have tried many times but still keep failing.
You should replace your personalisation string with %%BASEFILENAME_FROM_TRIGGER%%.csv - this is IMHO the most solid solution, as you avoid issues with time zones etc. you can get more details here