When the File Browser wants to open a file, it does so through the Exchange
Manager. Applications typically receive sysAppLaunchCmdExgReceiveData to open
files. The application won’t receive a sysAppLaunchCmdExgAskUser sublaunch but will
receive a sysAppLaunchCmdExgReceiveData sublaunch, just as it would for incoming
beams.
Applications should check the name field in the socket to see if it is a “file:” URL. If so,
the application should set the goToCreator field in the socket to its own creator ID.
It can then proceed as if a beam was received, but it shouldn’t create a new record.
Instead, it should store the parsed data somewhere temporarily and set the
goToParams struct in the socket to refer to this temporary area.

The wrapper function is as follows:
static Char *ParseFileURL (const Char *url, UInt16 *volRefNumP)
{
Char *path = NULL;
UInt16 refNum;
Err err;
err = SysLibFind(kFileBrowserLibName, &refNum);
ErrFatalDisplayIf(err, "Can't find file browser lib");
err = FileBrowserLibOpen(refNum);
ErrFatalDisplayIf(err, "Can't open file browser lib");
FileBrowserLibParseFileURL(refNum, url, volRefNumP, &path);
FileBrowserLibClose(refNum);
return path; 
}