Google is afraid once again for security reasons and restricts the type of files in attachments.
An example of this is apk files.
I suspect he is not the only one of this type and I did not clutter a list of accepted or rejected file types.
Here’s how I created a file of this kind that I received in an attempt to add it to a mail.
The help link comes with summary information without a clear boundary of the issue:
File types blocked in Gmail
There are a number of reasons why you may see the “This message was blocked because its content presents a potential security issue” error in Gmail. Gmail blocks messages that may spread viruses, like messages that include executable files or certain links.