Up to now I could find attachments inside brain without problems by searching for the name of the document. Do I miss some additional feature of your way to "mark" the filename?
Thanks Hannes for pointing that out. My suggestion of including the ID was to make the filename unique by reducing false positives, and also to speed up going from the file to the thought. For example, if the user happened to have a template file named myideas.txt which he would copy and attach to whatever thought needed an extra file attachment to hold his ramblings, then internal attachment to that particular thought would be sufficient to distinguish them. But if multiple files all named myideas.txt were to be housed in the same external folder there would have to be some other method for making them unique. For a start, directory structure forbids two files with the same name.
Three solutions which come to mind are:
1) Making sure the filename is unique without requiring including a code such as the ID. This is your suggestion, and also the simplest as the file manager (e.g., Windows Explorer) would prevent you from creating a file with the same name as an existing file. The only downside is that you might have to type in, or copy and paste, a considerable part of the filename into Extended or Advanced Search to prevent many false positive results, whereas typing a 4-digit number (the ID) is much more likely to be unique.
2) Marking the filename by including a unique character string (my suggestion) - why not the thought's ID to help locate it? Then you could have files named 2037 myideas.txt, 5028 myideas.txt, etc. A possible extra problem is that two or more thoughts might need to access the same external file, so maybe that file would need to include more than one ID in its name. It depends how much extra effort you would want to put into adding such a code, though with a little programming skill you might be able to create a macro to relieve you of all this drudgery!
3) Creating a separate subfolder named after the ID to house the external file. E.g., within the main folder there might be ones named 2037, 5028, etc., each containing myideas.txt. Again, a little more work required.
There must be more ideas that can be applied here so more suggestions are welcome. It's important not to make the solution too complicated, otherwise maintenance becomes a chore.