The /var/tmp folder has /dev/da0s3d mounted to it with 311M of free space. I have pasted the output of "df -h" from my ex4200 below:
root@juniper:RE:0% df -h Filesystem Size Used Avail Capacity Mounted on /dev/da0s1a 183M 152M 16M 91% / devfs 1.0K 1.0K 0B 100% /dev /dev/md0 281M 281M 0B 100% /packages/mnt/junos /dev/md1 6.8M 2.1M 4.1M 33% /packages/mfs-fips-mode-powerpc /dev/md2 5.4M 5.4M 0B 100% /packages/mnt/fips-mode-powerpc-15.1R6.7 /dev/md3 45M 40M 1.4M 97% /packages/mfs-junos-ex-4200 /dev/md4 83M 83M 0B 100% /packages/mnt/junos-ex-4200-15.1R6.7 /dev/md5 14M 9.0M 3.5M 72% /packages/mfs-jweb-ex /dev/md6 26M 26M 0B 100% /packages/mnt/jweb-ex-15.1R6.7 /dev/da0s3e 123M 4.9M 108M 4% /var /dev/md7 252M 10K 232M 0% /tmp/dev/da0s3d 369M 29M 311M 8% /var/tmp /dev/da0s4d 62M 124K 57M 0% /config /dev/md8 118M 17M 91M 16% /var/rundb procfs 4.0K 4.0K 0B 100% /proc /var/jail/etc 123M 4.9M 108M 4% /packages/mnt/jweb-ex-15.1R6.7/jail/var/etc /var/jail/run 123M 4.9M 108M 4% /packages/mnt/jweb-ex-15.1R6.7/jail/var/run /var/jail/tmp 123M 4.9M 108M 4% /packages/mnt/jweb-ex-15.1R6.7/jail/var/tmp /var/tmp 369M 29M 311M 8% /packages/mnt/jweb-ex-15.1R6.7/jail/var/tmp/uploads devfs 1.0K 1.0K 0B 100% /packages/mnt/jweb-ex-15.1R6.7/jail/dev /var/jail/jweb-app 123M 4.9M 108M 4% /packages/mnt/jweb-ex-15.1R6.7/jail/var/jweb-app /dev/md9 6.8M 2.1M 4.1M 33% /packages/mfs-fips-mode-powerpc /dev/md10 45M 40M 1.4M 97% /packages/mfs-junos-ex-4200 /dev/md11 14M 9.0M 3.5M 72% /packages/mfs-jweb-ex