If you set the maximum attachment size to something smallish (in my example 200k) and then try to upload a largish plugin (in my example, somewhere over 300k) then the plugin upload dies and reports the following action error message.

The following error(s) occurred:

• com.atlassian.confluence.util.FileUploadUtils$FileUploadException

I'm not sure which is the error though, whether its the max attachment size affecting plugin uploading (I can see how that would be by design even though its not obvious that it would be the case), or that GeneralUtil.getUploadFileTooLargeMessage isnt substituting the exception into the error message ... take your pick!