Add initial checks for file type and size

[kajalpatelinfo]

This PR adds checks for file type and size for uploads. It throws a 400 BadRequest when a file that does not adhere to these constraints is uploaded. It is beyond the scope of this PR to catch the Exception and handle it on the frontend.

The constraints are as follows.

• The file type must be a pdf, docx, png, or jpeg. docx can be removed if it is deemed so.
• The file must be under 10MB.

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	64.73% (-1.93% 🔻)	402/621
🔴	Branches	14.93% (-2.36% 🔻)	10/67
🔴	Functions	28.24% (-1.65% 🔻)	24/85
🟡	Lines	62.12% (-2.25% 🔻)	346/557

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`...` / wallet.service.ts

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`...` / attendees.controller.ts

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`...` / events.controller.ts

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Test suite run success

16 tests passing in 12 suites.

Report generated by 🧪jest coverage report action from 23ca04b0453e874e331f2e74a8d6e0bd813943a5

[xuxey]

Also good news -- The latest changes I pushed to the registration form will handle the frontend side of things :)