It sounds like an arbitrary limit set by some engineer without too much thought and without the incentive to document their decision. I do this sometimes as wel

It sounds like an arbitrary limit set by some engineer without too much thought and without the incentive to document their decision. I do this sometimes as well when dealing with untrusted data. Sometimes you don't want to have to think about the knock on effects of supporting large file sizes and it's easier to just tell the client please don't do that.