Jfirmware images downloaded from Juniper Portal:

https://support.juniper.net/support/downloads/?p=qfx5120

Following Images were found to be corrupt:
qfx-5e-jfirmware-25.4R1-S2.3.tgz 
qfx-5e-jfirmware-25.4R1-S1.4.tgz 
qfx-5e-jfirmware-25.4R1.12.tgz

The checksums are fine, i.e MD5 (qfx-5e-jfirmware-25.4R1-S2.3.tgz) = 96fe526884487ccc14e21edecc71995d

Tried in lab setup to upgrade firmware:

user@router> ...tmp/qfx-5e-jfirmware-25.4R1-S2.3.tgz
MD5 (/var/tmp/qfx-5e-jfirmware-25.4R1-S2.3.tgz) = 96fe526884487ccc14e21edecc71995d

user@router> request system firmware upgrade jfirmware fpga tmc file /var/tmp/qfx-5e-jfirmware-25.4R1-S2.3.tgz
Error: Corrupted jfirmware package.Signature verification failed.
Validating jfirmware package failed

Fix will be in 25.4R2 and further releases.

Corrupt files will be removed from the portal by the concerned team.