---------START-HTTP---------
POST /20180419/teams/noteam/channels/u7tbqcyetpds8r481geswme3gy/users/1aujpshixbbutkcpgu6yqq1xfa/xnzh4gghw3rtzrae3c9efn63no/test.docx?uploads= HTTP/1.1
(POST details truncated)
---------END-HTTP---------
fatal error: runtime: out of memory
runtime stack:
(stack trace truncated)
Bet you’ll never guess what the problem was! 
This is a pretty small server that I maintain for a small group of friends and only runs on about half a GB of memory. I added a 4 GB swapfile to the server - it didn’t have one previously. With the extra available memory, uploads are again working.
I did notice that image uploads do take a lot longer to process. Not entirely surprising, given the performance penalty from using hard drive space for RAM.
I am still curious to know why this broke after the upgrade, though. It seems unlikely to be a coincidence that I started seeing out of memory errors during upload at the same time I upgraded, doesn’t it? And it also seems strange that even small images (<100 KB) were causing this problem and continue to take several seconds to process, even with the swapfile speed penalty. Did anything change with the way file uploads are handled?