A simple analysis of the file path as a string won't always work. File paths can be constructed such that directory components have periods in them, e.g.,

C:\MyDirectory\LooksLikeAFileNameButIsReallyADirectory.txt

A simple string analysis can't tell if this is a directory path or full file name path. System calls are needed to determine the difference and this opens up an authentication can of worms...

Also, there's the whole UNC scenario with leading \\ServerName\ShareName\...