I was recently working with a customer who saw a 413 - Request too large in the Developer Tools > Console tab in their browser when uploading a plugin JAR ~3.9MB in size.
They initially thought this was a restriction on Morpheus’ end but I can confirm that this is not the case. It turns out there was an issue on their domain controller that was setting a restriction policy on the upload of files of a certain size.
If you encounter this error in the future and get confused by no output in the Morpheus logs, it is worth checking the Developer Tools > Console tab. You can then check internally whether there are any restrictions at the domain controller level (your infrastructure/security team should be able to answer this). Once the restriction is removed, you should be able to upload the JAR without any issues.
If anyone is interested in writing a plugin, do not be afraid! We have a lot of helpful documentation to get you started: