This may not be welcome information, but this is expected behavior.
When you drag and drop into a container field in Filemaker 12, the drag and drop is the same as using Insert Picture to insert a copy of the file into the container field. It does not insert a reference to that file.
The only times, in FileMaker 12, that you can open a file in a container field by double clicking the container field is if:
Insert File is used to insert the file with the "Store a Reference" option specified.
The field is not optimized for interactive content.
External storage has not been specified for the container field.
Depart from those requirements and you have to use Export Field Contents to open the file.
Drag and Drop on Windows in FileMaker 11, did produce something that could be opened with a double click. This worked because in FileMaker 11, this was the equivalent of Insert Object--which used OLE (object linking and embedding) to insert a link to the file. FileMaker 12 does not support OLE so this has resulted in a change in behavior.
One work around is to script a button to use Export Field Contents to open the file.
Another trick is to use the OnObjectModify trigger to perform a script that exports the inserted file to a specified storage location and then re-inserts the file with Insert File | Store a Reference specified so that now the file may be opened with a double click.
Just a little clarification to be accurate, even that will no change the sense of your comment nor your good tips and tricks :
External storage specified on a field has no influence if the file is inserted as a reference.
You are correct, but it seemed to keep the file from opening via double click when I tested it. But that was before I figured out that external storage should be used with "no reference" insertions... So maybe I didn't take a close enough look at that spedific combination of factors..