Currently, there is no way to select a single file within a folder. LinkFixer Advanced was designed to process huge numbers of files during a migration. So, we designed LinkFixer Advanced so that you can select drive, folders, and all of the files contained in a single folder.
We hardly ever find anyone who needs to process only a single file in a single folder.
The only way to process a single file would be to put that single file into a folder by itself, or to (temporarily) remove the other files from the folder that the subject file is in. Then, click the “files” box for the folder that contains that one file.
It is possible that we will change the user interface to allow the selection of a single file. But, so far, we have gotten so few requests for this feature that it has not been a high priority. If this is something you need for your project, please contact support so that we can use your feedback to prioritize this feature.