Just tried that. It doesnt seem to create any kind of file. it also reads out all zeros for every block:
[usb] pm3 → lf t5 dump -f my_lf_dump
[+] Reading Page 0:
[+] blk | hex data | binary | ascii
[+] ----±---------±---------------------------------±------
[+] 00 | 00000000 | 00000000000000000000000000000000 | …
[+] 02 | 00000000 | 00000000000000000000000000000000 | …
[+] 03 | 00000000 | 00000000000000000000000000000000 | …
[+] 04 | 00000000 | 00000000000000000000000000000000 | …
[+] 05 | 00000000 | 00000000000000000000000000000000 | …
[+] 06 | 00000000 | 00000000000000000000000000000000 | …
[+] 07 | 00000000 | 00000000000000000000000000000000 | …
[+] Reading Page 1:
[+] blk | hex data | binary | ascii
[+] ----±---------±---------------------------------±------
[+] 00 | 00000000 | 00000000000000000000000000000000 | …
[+] 01 | 00000000 | 00000000000000000000000000000000 | …
[+] 02 | 00000000 | 00000000000000000000000000000000 | …
[+] 03 | 00000000 | 00000000000000000000000000000000 | …