Hi everyone,
I’m currently merging an old Salesforce Account to an old HubSpot Account and then migrating all the data to a new Hub.
Now I want to upload files and associate them to the according Contact, Company or Deal. What is the best way to do this?
Status Quo: I have all the files and a CSV with filenames and the Record ID / Emails of the new Hub
Thanks in advance! 
Hey @waffle and a big warm welcome to the HubSpot Community! 
Thank you for reaching out and sharing the details of your migration project, that sounds like a big but exciting task! 
Let’s consult our Top Experts: Hi @Anton, @evaldas and @SteveHTM do you have any tips to help @waffle, please?
Thanks so much and have a lovely day!
Bérangère
@waffle - Definitely an interesting challenge! I’ve done quite a bit of work with automation of file file handling and manipulation of uploads to associate them with various records. So I know that, at least at the API level, such automation is unquestionably feasible.
You don’t give any details if your new Hub profile, but API work - through custom workflow steps - would need Data hub. Also - and I think this is key for a process of any scale - you may need a mechanism to upload your master list CSV records which would then be useed as a way to drive subsequent file processing automation. For this you could use custom objects or HubDB. I suppose it would also be possible to use some kind of external scripting environment that can invoke API calls, but I’m typically inclines to keep processes inside HubSpot.
Best of luck with this. If I can offer any more detailed help please reach out.
Steve