Produce a warning prompt when users wishes to download a potentially dangerous file

[martgil]

Description:
Currently, when a user downloads a potentially dangerous file type, the app allows them to do so without any warning.

For improvement, we add a user warning just when the user decides to download a potentially dangerous file. I don't have a list for the ios app to block maybe except the .ipa file for the moment. But I will gather more information and add it here.

For the message, we use the message from FlowCrypt/flowcrypt-browser#4862 for uniformity:
This executable file was not checked for viruses, and may be dangerous to download or run. Proceed anyway?

Reference: https://github.com/FlowCrypt/flowcrypt-security/issues/224