I recently heard from a client saying their blog was loading very slowly. The issue was caused from a high resolution image in .tiff format which is generally used for print. In order to block these file types add the following plugin to your mu-plugins folder.
/* prevent uploading of .tif files.
Full list of mime types
add_filter( 'upload_mimes', 'theme_restrict_mime_types' );
function theme_restrict_mime_types( $mime_types )
$mime_types = array(
'pdf' => 'application/pdf',
'doc|docx' => 'application/msword',
'jpg|jpeg' => 'image/jpeg',
'gif' => 'image/gif',
'png' => 'image/png'
save as restrict-upload-types.php and upload to your mu-plugins folder.