From the Release Notes for 12.5
Endpoint Known Issues:
1890889:
Incidents are not created for Recipient URL conditions for webmail services, such as gmail.com or yahoo.com.
Workaround: None.
Tejas,
From what I understand, the issue with some of these webmail services is exaclty what you noted, they are offloading a portion of the work to different services. That is, you upload a file, but the browser actually does not do the job, it is passed to a child service (sometimes windows services), and delivered.
Engineering is aware of the issue, hence the "Known Issue". Hope this helps!
Joseph