.msg files could be all sorts of files. You should handle that as Interactive content. So far so good.
Do you know what size the .msg file is? From my experience FileMaker does not handle different file types differently when drag and drop into a container field.
File size is normal, just around 200 KB. Even the pdf twice or thrice this size work pretty good.
I have tried setting the field for Interactive content but it doesnt help and neither the conventional "Insert File" method.
I have just realized that it does the same with other office documents as well, even with documents less than 100 KB (word, excel ) it takes around 20 - 30 seconds by going into "Not Responding" mode and then finally inserting the file.
Did it use to work for older versions?? I am using office 2013 suite.