There are two ways of selecting a file to be placed by addfile: Direct and Dynamic. Display the addfile plugin dialog (see To attach a plugin to a field) then choose:

Click to expand/collapse textDirect

Select the Direct tab of the Configure dialog, then

1.Enter or Browse to select the file.
2.Select/enter the number of the file-page from the spin-box at the foot of the dialog.

See File placement to set up the positioning of the file-page.

Click to expand/collapse textDynamic

Select the Dynamic tab of the Configure dialog. Set up a template for selecting a file using the value of the field that called addfile:

1.Enter a prefix, typically the path of the data-file.  This may be left blank if a prefix is not required.
2.Enter a suffix, typically the file-extension.  Default value .PDF.
AddFile constructs a new file-name by concatenating the prefix, the text extracted from the data-field and the suffix.  This allows a different file to be placed when the data changes.
See also example file-name in Image selection.
3.Select/enter the number of the file-page from the spin-box at the foot of the dialog.

See File placement to set up the positioning of the file-page.