First of all, by "key", I assume you mean primary key.

I'm also assuming that there's no guarantee of uniqueness in your filenames, or even directory names.

If you were confident that the directory2/file.ext combination would be unique for each file, I'd parse those out into two fields, filename and directory, and use the combination as primary key. That way you wouldn't have to use TEXT, which is an understandable desire.

If not, using the full path is probably your best bet, unless you wanted to just use an index, but that's usually cumbersome.