Add safeguards for attachment size

[leplatrem]

By default, prevent attachments to be more than X MB.
Then enable bigger attachments on use-case that are justified.

[leplatrem]

What would be the size limit?

As JSON records, we recommend in our onboarding docs not to go over 1MB in total for the collection.

As attachments on records, beyond CDN costs, there is no hard limit.

It all depends on how the client feature is implemented. For example:

• if the attachments are downloaded in background, without user content, or as a result of a user action and/or with UI feedback
• downloading a 100MB file on fiber internet and SSD hard drive, is very different than with DSL and slow hardware
• downloading files on startup, or interferring with user browsing, is very different than during idle times
• downloading a big file for an optional/advanced feature with low update frequency can be more acceptable than a smaller file changing every day

That being said, beyond 20MB per file our CDN caching requires some specific configuration to enable segments. And as of today, this concerns only 2 collections. We could arbitrarily decide that our limit is 20MB, and disable this limit when the use-case justifies it.