Allow setting of public/private for files to be a parameter

[jwrober]

From here - https://discuss.erpnext.com/t/default-file-setting-to-private/23698

Would like to have a setting of some kind to be able to set the default behavior of files. In my use case I want the default to be private. However others might want public. So having a global setting run this would be great.

[Tropicalrambler]

This is CRITICAL.

[bcornwellmott]

I made a PR to have the default as private, because it's a more robust condition than making things public by default. Adding a setting will probably be harder.
frappe/frappe#3521

[tundebabzy]

Merged

[cosimot]

now it is set as public by default again.

[mmasifashraf]

its came as public by default again

[Lewinta]

I suggest to include an option for Attach fieldtypes on Customize Forms where users can specify if this check is ready only and allow to choose a default (0 or 1)