Capitalize file extentions result on skipping static pre analysis

[Cryss76]

static analysis is skipped if the file extension is on capital letters

Steps to reproduce the behavior:
get a PE-file and change the .exe-extension to .EXE,
submit the modified file
after analysis: no static analysis result, like "PE file", are shown in the report.

Expected behavior:
Some users/developers set a capitalize extension on their files.
Therefore static analysis shouldn't be case sensitive on the extension
and normally perform the analysis.

Cuckoo 3.0.1, Ubuntu 20.04

Fix:
on this line add lower() after filename