As per our engineers, there was an issue before (even in 1.5.x) where PDF files did not have the “.pdf” extension added which resulted in issues opening the file locally. What we did to address issues like that was to append extensions to the files that didn’t have it based on their MIME types.
(Extension added to files based on its MIME type)
I’d like to request a way to switch off this user friendliness 
Hi @pedro
Let me forward this to our engineers. Thank you for raising this to my attention!
Any news?
Hi Pedro,
My apologies, but we have not been able to investigate this feature request. To give you an idea of how feature requests are made for Insync, we base this on the number of requests from users + frequency of requests. When we have reached the right numbers, we then move the request for research and development.
Thank you for understanding.
Just came across this issue. In my case I have a web app and on the server I store PDF files without extensions. I will FTP copy my files from the server to my local workstation. From the local workstation they are pushed out to my Google Drive for safe keeping.
If I use Insync the files back to another workstation, the PDF extension is added, thus resulting in my code not being able to find the files.
Spent hours on what is going on and I just found that it was Insync adding extensions. Not cool.
Evalute if rclone works for you. Is a bit different than Insync but may work for your usecases. No transformation is made in mount mode AFAIK. On copy mode it’s optional.