So ...

Upon activation, access the Settings => Discussion page and scroll down to the "Comment Attachment" section:

This is where you can manage the settings to display an image attachment field in the comments section of your Wordpress posts and pages.

You can customize the the position of the comment field, the field title, the required option, the allowed file types, the image attachment size, and more.

More than that, you can also allow your users to attach all kind of documents (PDF, DOC, PPT, XLS, etc), archive files (RAR, ZIP), audio files (MP3, M4A, OGG, WAV, WMA) and video files (MP4, M4V, MOV, WMV, AVI, MPG, OGV, 3GP, 3G2, FLV, WEBM).

All you have to do is to check the file type options in the comment attachment settings:

And of course, don't forget to click the "Save Changes" button ...

OK. Now let's see the result ...