How wordpress java determines mime types of uploaded media

Introduction

This is actually about how Java determines mime types of files.

Details

As explained in here, mime types in java are determined in such a manner:

1. The file .mime.types in the user's home directory.
2. The file <java.home>/lib/mime.types.
3. The file or resource named META-INF/mime.types.
4. The file or resource named META-INF/mimetypes.default.

So, if you upload a file (say, a png file) and the mime type appears as

type:application/octet-stream

Just create a file ~/.mime.types and add an entry like this:

image/png png

Which should fix it.

A more general solution would be copying /etc/mime.types to ~/.mime.types, which contains a huge amount of file extensions.