How to download file in browser from Go server

Question

My code get file from remote url and download file in browser:

func Index(w http.ResponseWriter, r *http.Request) {
    url := \"http://upload.wikimedia.org/         


        

Answer 1

What you need to use is the Content-Disposition header with attachment value. MDN web docs says the following about the header:

[...], the Content-Disposition response header is a header indicating if the content is expected to be displayed inline in the browser, that is, as a Web page or as part of a Web page, or as an attachment, that is downloaded and saved locally.

If you want to specify a filename for the clients you can use the filename directive. In the following format:

Content-Disposition: attachment; filename="filename.jpg"

This is an optional parameter and it has some restrictions.

[...] The filename is always optional and must not be used blindly by the application: path information should be stripped, and conversion to the server file system rules should be done. [...]

To format filename properly in the header you should use mime.FormatMediaType. Example:

cd := mime.FormatMediaType("attachment", map[string]string{"filename": d.Name()})
w.Header().Set("Content-Disposition", cd)

In this case for content type you can use application/octet-stream because the browser does not have to know the MIME type of the response.

[...] Generic binary data (or binary data whose true type is unknown) is application/octet-stream. [...]

w.Header().Set("Content-Type", "application/octet-stream")

For outputing the content of a. file I would recommend to use http.ServeContent or http.ServeFile because they handle RFC 7233 - Range Requests out of the box. Example:

f, err := fs.Open(name)
// [...]
cd := mime.FormatMediaType("attachment", map[string]string{"filename": d.Name()})
w.Header().Set("Content-Disposition", cd)
w.Header().Set("Content-Type", "application/octet-stream")
http.ServeContent(w, r, d.Name(), d.ModTime(), f)