To see what filepath exists for a given container field, define a calculation field that returns text and simply enter the name of the container field as the sole term in this calculation field's expression. Put the field next to the container field and size it tall enough to display at least three lines of text. You'll see the filepath in the third row of text.
Is it possible to make a relative path to a referenced file, f. ex. to a subfolder in the same folder as the FileMaker file?
If the file is stored in teh same folder as the database file and you know the filename, you can define a calculation field set to return "container" as its field type that combines the Filename entered in a text field with the results of Get ( FilePath ) to construct the filepath to your file. Other Get functions can return the filepath to your desktop or the temporary items folder.
"File: " & Get ( FilePath ) & FileNameField
Is one possibility, but other forms such as "image: " & ... for picture files are also possible.