certificate_generator/app/routes.py: fix for potential path manipulation

[seowwj]

Pull Request Template

script name - certificate_generator/app/routes.py

Brief about script
Fixes potential path manipulation in the routes.py file.

• Checks if the filename is alphanumeric
• Checks if the filename does not contain '/' or '..'

Issue no.(must) - #951

Self Check(Tick After Making pull Request)

• This issue was assigned to me.
• One Change in one Pull Request
• My file is in proper folder (Name of folder should be in lowercase with no space in between) (E.g. meet_schedular)
• I am following clean code and Documentation and my code is well linted with flake8.
• I have added README.md and requirements.txt (Include version numbers too e.g. pandas==0.0.1) with my script
• I have used REPO README TEAMPLATE (Necessary)
• Just including required dependencies in requirements.txt (Don't include Python version too)

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