Yes, you can write your own trigger on the Account object along side any Managed triggers that are there on the object too.
insert of an
Account record you can check for the value of
YES, query for related
Attachment records and throw an error when none are found but that is the extent of it.
There will never be an
Attachment related to the
Account during an
insert operation because the
Account needs to exist prior to associating the
Attachment to it.
You would be better off to skip a query for the attachments and simply prevent the insert of an Account with this picklist having a value of
YES. This can be done with a Validation Rule as well as in trigger code.
update of the
Account object your scenario becomes more valuable.
The trigger code during
update could inspect the related attachments and ensure that the user had uploaded an
Attachment prior to changing the picklist value to
YES and saving the