The whole application was tested by 1 QA who was uploading only one 3MB file.
Now on production ~300 users upload in a batch one hundred 100 MB files... and nothing works now
WHO WOULD EXPECT THAT?

I guess the use case analysis and the requirements that followed were not exact enough.

Apparently not QA. 🤣

Maybe the QA was instructed against attempting stress tests and instead told to focus on validating functionality. And if the 3MB file worked, the functionality was validated. I'm QA and I have a folder of aberrant files (corrupt, abnormally large, etc) and I've had quite the choice of words being used on me by reporting defects using those files to break the system under test. And to be frank they're right under ISTQB procedures stress testing is not functional testing. Unless explicitly told the QA is going to validate requirements not the robustness of the system. Was such need part of the test plan?

For your QA:
Generate test files in load/stress/performance tests. Throw it in a lightweight Docker container running Bash and mount the file system to generate files in the source path.

for FILE in *.full ; do split -l 100000 $FILE; mv -f xaa `echo "$FILE" | cut -d'.' -f1`.txt; rm -f x*; done

List open processes by #files opened and monitor the application.

ps -ef |awk '{ print $2 }' \ |tail -n +2 \ |while read pid; do echo "$pid$(lsof -p $pid |wc -l)"; done \ |sort -r -n -k 2 \ |while read pid count; do echo "$pid$count$(ps -o command= -p $pid)"; done