I have a document library with a few site collection specific extra columns. Uploaded msg files present the dialog with the Open button when clicked.
I save the document library as a template and it is available as a solution.
I add a few other lists to the site and save the whole site as a template, which is also available as a solution.
When I create a site from that site template and upload a few emails (.msg) files to the document library, the Open button does NOT appear.
I then created a new document library from the library template mentioned above. Emails uploaded to that library show the Open button just fine.
I've been through all the GUI settings of the two document libraries with a fine tooth comb. I cannot see any difference between them.
Does anyone have an idea what could be causing the difference? Obviously, the Mime type is registered. I can reproduce this behaviour with other sites created from the site template.