Just want to add my vote for removing this limit.
I frankly think this is not appropriate for a premium service where you take good money for. The total of 20 GB should be enough protection against misuse.
I still get those warnings from time to time even when I am not aware of having added internal attachments. In my case I have 0.0 GB (!) Internal Attachments Size according to statistics. I just want to get rid of those annoying messages.
I do not use TheBrain for data storage. (Dropbox has better features for this purpose like unlimited versioning and fast LAN update between local computers.)
BTW: How could I get a list of internal attachments including the amount of memory they take? The only list I know of is the report on internal attachments.