feat(example-file-upload): add an example for file upload with LoopBack 4

[raymondfeng]

An example app for file upload.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[dhmlau]

Thanks @raymondfeng for creating this example!
I wonder if it would be better to somehow show what got uploaded even it's in memory. I tried the example but I don't know a way to verify it actually got uploaded, or maybe I'm just missing something. :)

[bajtos]

I wonder if it would be better to somehow show what got uploaded even it's in memory. I tried the example but I don't know a way to verify it actually got uploaded

+1 to add a way for inspecting the uploaded files

[raymondfeng]

I switch to diskStorage as the default now. Uploaded files will show up in .sandbox folder under the application root directory.

[dhmlau]

LGTM once the upload location is updated in code or README.

[hacksparrow]

A suggestion for future example apps. The changes required for the example should be in a separate commit, so we have a good visibility about the amount of changes that goes into making the feature work.

[jannyHou]

Tested on local LGTM 👍