I have run into this issue several times and I would like to question the design decision that has lead to this "feature": Whenever I drag a file and drop it in the lower part of the composing window, Thunderbird only inserts the file *name* into my message. I would like Thunderbird to *attach* the file regardless of the drop area. My recommendation: Leave the filename thing out. If you, by all means, think that people will need this feature, just do both: Insert the filename into the message AND attach the file.
See bug 452092 for a more comprehensive discussion on this issue. Drag-and-drop operations should have a certain semantics that's intuitive to the user. I agree that just inserting a name or path of the file isn't likely what the user wants. Which platform are you seeing this on? It appears to me that nothing is actually happening when trying to drag a file into the composition window, using Windows 7.
Thank you for referring me to the related bug. I will look into it later today. I encounter this issue on Linux (FWIW: x86_64, Ubuntu 11.10, Kernel 3.0)