webpack-dev-server proxy doesnt work

Here is the part of my webpack config:

devServer: {
port: 4200,
historyApiFallback: true,
proxy: { "/file/**": { "target": proxyFileUri, "changeOrigin": true, "pathRewrite": {"^/file" : ""}, "secure": false }, "/api/**": { "target": proxyApiUri, "changeOrigin": true, "secure": false }
}

},

While trying to proxy all requests by webpack-dev-serv somthing went wrong. What is the problem? I can not understand why the proxy does not work on the key "/file", path doesnt rewrite, i can't find a solution. Could the problem be that I'm sending a request with "Content-type : multipart form data"?