当 MIME 类型为空时，Rails Active Storage 会阻止上传

Question

I'm uploading a file using Direct Upload and Active Storage in Rails 6. When uploading a .stl file I am met with the following error Mime::Type::InvalidMimeType ("" is not a valid MIME type) which results in a 406 error.

When checking the file with JS it does also not return a Mime type. The file type is simply empty.

This is the case for all .stl files I've tried. Image files work as expected.

I've added the correct Mimetype to a rails initializer using Mime::Type.register "model/stl", :stl . But of course this makes no difference.

The file is submitted in JS using DirectUpload.

  new DirectUpload(file, url, this)

There is no file validation.

So essentially my questions are;

• Why is the mimetype empty?
• Is there a different way of verifying the filetype?
• Can I somehow set the mimetype without causing security issues?
• Can I somehow accept the file anyway, even if the mimetype is empty?

Answer 1

So, the problem seems to be that ActiveStorage uses javascripts File.type method to determine the mime type, which is not too reliable https://developer.mozilla.org/en-US/docs/Web/API/File/type (it only checks the extension and it looks like it depends on the operating system's settings).

Since you already know what mime type you want the file to be and given that it already uses a not too reliable method, I guess it's ok to set the file type manually if the file extension is the one you are expeting to be.

In order to override the original direct upload action, you can create a new controller to extend the original https://github.com/rails/rails/blob/master/activestorage/app/controllers/active_storage/direct_uploads_controller.rb . On your custom controller override the create action and set the value of params[:blob][:content_type] if it's empty and then just do the same or call super.

For DirectUpload to use your controller and action, you need to change the route that direct upload uses https://github.com/rails/rails/blob/master/activestorage/config/routes.rb . Just add a new route for your controller using the same url /direct_upload .

Answer 2

Just to clarify the closing paragraph of arieljuod's answer:

The source route is prefixed; your overriding route must include the full path. For instance, to override the default create action:

post "/rails/active_storage/direct_uploads" => "your_controller_name#create"