[TS][Angular] fix multiple file upload

Created by: topce

for typescript-angular client https://swagger.io/docs/specification/describing-request-body/file-upload/ fix Multiple File Upload it was broken for typescript-angular client from link above

Use the multipart media type to define uploading an arbitrary number of files (an array of files): requestBody: content: multipart/form-data: schema: type: object properties: filename: type: array items: type: string format: binary was broken did not generated proper HTTP request

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)