[cpp-qt-client] Fix files

• foreach => for
• Whitespace formatting
• Fix files

I fixed the compilation for an upload operation with the following path:

      "post": {
        "tags": [
          "Files"
        ],
        "summary": "Upload files",
        "description": "The language and password provided will be applied to all files in the request",
        "operationId": "Upload",
        "parameters": [
          {
            "name": "language",
            "in": "query",
            "required": true,
            "schema": {
              "type": "string"
            }
          }
        ],
        "requestBody": {
          "content": {
            "multipart/form-data": {
              "schema": {
                "required": [
                  "files"
                ],
                "type": "object",
                "properties": {
                  "files": {
                    "type": "array",
                    "items": {
                      "type": "string",
                      "format": "binary"
                    }
                  },
                  "password": {
                    "type": "string"
                  }
                }
              },
              "encoding": {
                "files": {
                  "style": "form"
                },
                "password": {
                  "style": "form"
                }
              }
            }
          }
        },
        "responses": {
          "201": {
            "description": "Created",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "object",
                  "additionalProperties": {
                    "type": "array",
                    "items": {
                      "type": "string"
                    }
                  }
                }
              },
              "application/json": {
                "schema": {
                  "type": "object",
                  "additionalProperties": {
                    "type": "array",
                    "items": {
                      "type": "string"
                    }
                  }
                }
              },
              "text/json": {
                "schema": {
                  "type": "object",
                  "additionalProperties": {
                    "type": "array",
                    "items": {
                      "type": "string"
                    }
                  }
                }
              }
            }
          },
          "403": {
            "description": "Forbidden"
          }
        }
      },

I'm not totally sure of my fix but I deliver an example that is compiling and would be happy to have your feedback!

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cc @ravinikam @stkrwork @etherealjoy @muttleyxd