The previous rate limit allowed to post media so fast that it is possible
to fill up the disk space even before an administrator notices. The new
rate limit is configured so that it takes 24 hours to eat 10 gigabytes:
10 * 1024 / 8 / (24 * 60 / 30) = 27 (which rounded to 30)
The period is set long so that it does not prevent from attaching several
media to one post, which would happen in a short period. For example,
if the period is 5 minutes, the rate limit would be:
10 * 1024 / 8 / (24 * 60 / 5) = 4
This long period allows to lift the limit up.
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