Google is blocking the apk because it detects it as a virus even though it could be a false positive. You can compress the file first see you can upload it, or even encrypt the apk with a password so that you can attach it to Gmail. You can use WinRAR or 7zip for that.
How do I email an apk?
You cannot attach the file in your email directly, since APKs are treated as junk by most email providers. So upload the apk somewhere and send the link to your co-worker. If a link is not enough, with a simple trick, you can send the file anyway: Rename the file: attach .18 Jun 2010
What is the official app for Gmail?
Gmail is the official app for the Google email client that lets you manage you email account (and any other account you may have) using a clean and user-friendly interface.
What is a default email application?
The default mail program is the software that is registered with your operating system as the software that is to handle mailto URL's. When another application wants to send an email, it looks up the default and passes the email to the default mail program.27 Mar 2000