Gmail seems to not allow files of type extension .exe, .dll and so on which are executable. This is a reasonable step towards cribbing virus, but what about users who want to trade .net libraries? Well, initially I thought we can put in the needed files in a zipped format and then send them. That seemed to be a failed effort as gmail seems to scan inside the zip file contents. Next I tried to encrypt the zip file and then send it as an attachment, it also failed as gmail was able to enumerate the file names and then block the transfer. Finally, instead of compressing the content as zip, I did is as rar (winrar) and also encrypted the filenames in the rar file, only after doing that gmail allowed the transfer of file.
I personally think that currently users are quite aware of not to execute executable files that are sent over the email and it is kind of inconvenient to block the type of files that can be transferred.