GuessContentType
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.
Setup
Add the plugin to your server.
$server->addPlugin(new \Sabre\DAV\Browser\GuessContentType());
If you'd like to add more contenttypes, you can add values to the
Sabre\DAV\Browser\GuessContentType::$extensionMap
property.