The reason is stated in your first post:
I've set up the field to store the files externally
If you use the external storage option, you cannot open the file by double clicking the container field. You'll need to use Export Field Contents to export and open a copy of the file. In many cases, this is best done by exporting the file to the temporary folder so that an extra copy of the file is not left on the user's computer after they are done with the file.
And I readily agree that this method of opening the file is not ideal if you want to open the file, edit it and then save those changes back to the original copy saved in the external storage folder. To do that, you have to re-insert the copy that you exported after closing and saving changes to it. And this is leave the door open that the user might save changes to the copy but forget to click a "save" button or some such to perform the script that re-inserts the file in order to update the original copy.
Thanks - is there no way to store images so that they can be opened from Filemaker?
There are plugins that address file manipulation and accessing images.
Supercontainer is one such plugin. http://360works.com/supercontainer/
A list of file manipulation plugins http://filemaker-plugins.com/features/file-manipulation/
Of course they can, been doing it for years, but there are trade offs that can make this approach less than ideal as well.
Do not use external storage--you have to set up your own shared directory on a server if you have multiple users accessing your database.
Insert the file using the Insert File option with the "store a reference" option selected. (Drag and drop won't work.)
the field is not "optimized for interactive content"
You can double click the field and FIleMaker will cause your operating system to open the file with the specified default application for that file type.
Please note that this does not work for a web published database nor for users remotely accessing the database from outside of the local network. Each client computer also has to map/mount the shared directory in identical fashion (a problem when you have mac and windows machines both accessing your DB) All users that need to insert files must have "write" permission to the shared directory. This means that they can open and delete all files from this directory.
So you may find that Export Field Contents really is the better option even though there are significant limitations with that option.