I was trying to attach a 4.5MB archive yesterday and failed. Today I'm going to file a bug which is going to require me to attach log files which are going to be ~10MB each bzip2 compressed. Please increase the attachment size limit of bugzilla to something sensible, such as 20MB. Thanks!
Currently the limit is set to 4MB which explains the issue with your attachment. BMO development and IT feel like we are not yet capable of allowing attachments larger than 4MB at this time due to database limitation and performance. Other solutions are being investigated such as distributed filesystems where we can start storing attachment data instead of keeping it in the database. Until then we recommend storing the files outside of BMO and linking to them from the bug report.
with the new scl3 cluster we're now in a position to increase the attachment size limit.
maxattachmentsize set to 10240k.