In the folder <Installation Path>/wp-includes/ there is a file called functions.php, inside this file there is a function called get_allowed_mime_types().
This function is in charge of defining an Array of mime types keyed by the file extension with a regex corresponding to those types.
So in order to add a new file-type to the white-list you just have to add a new line with the following format in the list:
'jpg|jpeg|jpe' => 'image/jpeg',
Where, on the left you define the extensions you want to be accepted separated by the vertical bar character (“|”) and on the right you give a file definition.