Hiding or removing file extension displayed in HTML on attachment page

You could use substr()

Returns the portion of string specified by the start and length parameters.

in combination with strrpos()

Find the numeric position of the last occurrence of needle in the haystack string.

Example

$name = basename( wp_get_attachment_url( $post->ID ) );
echo substr( $name, 0, strrpos( $name, '.' ) );

This will show the string (in this case a filename) untill the last occurance of ..