By default SabreDAV is not aware of any contenttypes of any files. This is because there is no reliable, portable and fast way to determine the contenttype/mimetype of files.
The GuessContentType plugin guesses the contenttype of files by doing what everybody else does; look at the extension.
If this plugin is added, it will only add a contenttype header/property for files that are recognized and don't already supply a contenttype themselves.
Note that GuessContentType is only available in version 1.1.0 and later.
Add the plugin to your server.
If you'd like to add more contenttypes, you can add values to the