Guide
You can use File fields in automations freely.
File field trigger
If you want to react on new file addition or some file deletion, select Entity linked to a [Entity] or Entity unlinked from a [Entity] trigger and then select File field. You can filter events by File name. Maybe the most popular case is to filter by file extension.
Note that this trigger fires on every linked File. For example, if 10 photos will be linked to recipe, this rule will be executed 10 times
File field actions
There are several actions you can set for Files fields:
Add File to [File Field] from URL - add new file to the field by providing file name and URL
Delete[File field] - delete all files from file field
Attach PDF to [File Field] using Templates - generate a PDF and save it into files field. Check Generate PDF File using Button as an example
You can also copy Files from one entity to another. For example, you receive new Email from a potential candidate and copy all attached files to the relevant Candidate entity via formula.
Note that copy action will replace all existing files, it is pure copy of the whole collection. For example, in a rule in a screenshot below, Files field in Candidate database will be fully replaced by content of Files field from Email.
In Buttons you can set Ask User for Files fields, so a user can upload files during entity creation. It also means that a user can attach files manually in Send Email action (or select files to attach via a formula).