Two options I can think of...
1. Use the native "Insert File" command bring a file into a container field. Make sure to store as a reference. You can then use GetAsText (theContainer) to parse and format the file path.
2. Use a plugin, like Troifile (http://www.troi.com/software/fileplugin.html) or others that have file selection/manipulation capabilities.
Hope this helps.
The first option worked perfectly, by putting the text from the container field into a global variable and then using the global variable in the specify attachment open window. In fact it wasn’t necessary to either parse or concatenate more info into the text the variable retrieved because it already had a full path set of information in it.