I would say it depends on your workplace environment. If you are friendly and close with your boss and/or coworkers, it is okay to sometimes use contractions (but better to avoid them if you're unsure).
For the two sentences that you wrote, I'd suggest the following:
"I am sharing the file as the attached." -> "I have attached the file." OR "I have attached the file as requested." OR "The file is attached."
"Let us use this material." -> "Let's use this material." OR "Let's move forward with using this material."
If you're the boss and will not be a direct part of the project using the material, you can write, "Please use this material." OR "Please move forward with using this material."
In my experience as a native speaker, it's very rare for native speakers to use "Let us" except in extremely formal situations. It's better to use "Let's" here. It doesn't sound unprofessional; it sounds confident and inclusive of the team. That being said, you can always rephrase the sentence so you avoid using the contraction or phrase altogether.
혹시 궁금하신 게 따로 있으시면 물어보세요!