typescript-fetch: fix isCollectionFormatMulti for arrays of files

[m1k1o]

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Details

Similar problem as with typescript-axios in #8104, where isCollectionFormatMulti was not set properly for arrays with binary items.

Sample request body:

requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          files:
            type: array
            items:
              type: string
              format: binary

Incorrect handling before PR:

        if (requestParameters.files) {
            formParams.append('files', requestParameters.files.join(runtime.COLLECTION_FORMATS["csv"]));
        }

Correct handling after PR:

        if (requestParameters.files) {
            requestParameters.files.forEach((element) => {
                formParams.append('files', element as any);
            })
        }