I can’t confirm either, but I know Rails uses it, and DHH is from the PHP community, so that’s my assumption of where it comes from.

More importantly - post the entire form fragment when discussing problems with input elements. On MDN enctype="multipart/form-data" kind of sticks out like a sore thumb - but I figured that you had that covered given you got the PHP version working.

Aside: Does anybody have any information on the origin of the “[]” name suffix convention for multiple files? It seems that the HTML specification doesn’t require it but it seems to be more of a server-side convention (PHP specifically). Or am I wrong about this?

Only for standard HTTP upload. Is not WebSockets standard better for that? You can send multiple files at a time in their own small requests without sending HTTP headers, extra (every request) validation etc. It’s also better to for future changes like showing upload progress, chucking etc. Personally I don’t like also ideas of sending multiple files in one request. Application should work as long as it can. In your case if you would have a network problem in the middle of process then you would need to send again all of hundred or even few thousand files. Even if they are tiny it’s not good for user experience and it’s also not scalable solution. Remember that there are still people which do not have fast fiber internet.

I believe that @josevalim should know best this problem as he is most active contributor to plug. I think that you should create a GitHub issue for this problem in elixir-plug/plug. I did not looked at source code of plug, but if I would guess then I think that plug is trying to handle multiple upload as same as normal upload which means that it hits an edge case of file upload implementation in plug where developers does not expect to parse request content of multiple files. Also I believe that Issue when uploading a file with multipart/form-data with Chrome but trying to upload a directory instead of a file plug issue is really similar to your case (not properly handled request content standard). Again I recommend to create new issue with providing all data as in linked issue. Also if you would look on that issue then you would see that different browsers could send requests content in different format, so make sure to test them and give us information about it as in linked issue.

Yeah, just found that @josevalim commented about it 2 years ago:

It doesn’t seen the multiple option is working for file_input. So you need to do rather something like: file_input f, :files, name: "files[]" or send a PR so we support multiple.

This means that you need to have multiple file_input with name which ends with [] (because it shows Phoenix that we want to save value of multiple inputs into one list). [] suffix should not change parsing parameters for only one field. Just instead of value you would have [value] and nothing more, so it’s useless for you.