Source

File: wp-includes/customize/class-wp-customize-media-control.php

public function get_default_button_labels() {
		// Get just the mime type and strip the mime subtype if present.
		$mime_type = ! empty( $this->mime_type ) ? strtok( ltrim( $this->mime_type, '/' ), '/' ) : 'default';

		switch ( $mime_type ) {
			case 'video':
				return array(
					'select'       => __( 'Select video' ),
					'change'       => __( 'Change video' ),
					'default'      => __( 'Default' ),
					'remove'       => __( 'Remove' ),
					'placeholder'  => __( 'No video selected' ),
					'frame_title'  => __( 'Select video' ),
					'frame_button' => __( 'Choose video' ),
				);
			case 'audio':
				return array(
					'select'       => __( 'Select audio' ),
					'change'       => __( 'Change audio' ),
					'default'      => __( 'Default' ),
					'remove'       => __( 'Remove' ),
					'placeholder'  => __( 'No audio selected' ),
					'frame_title'  => __( 'Select audio' ),
					'frame_button' => __( 'Choose audio' ),
				);
			case 'image':
				return array(
					'select'       => __( 'Select image' ),
					'site_icon'    => __( 'Select site icon' ),
					'change'       => __( 'Change image' ),
					'default'      => __( 'Default' ),
					'remove'       => __( 'Remove' ),
					'placeholder'  => __( 'No image selected' ),
					'frame_title'  => __( 'Select image' ),
					'frame_button' => __( 'Choose image' ),
				);
			default:
				return array(
					'select'       => __( 'Select file' ),
					'change'       => __( 'Change file' ),
					'default'      => __( 'Default' ),
					'remove'       => __( 'Remove' ),
					'placeholder'  => __( 'No file selected' ),
					'frame_title'  => __( 'Select file' ),
					'frame_button' => __( 'Choose file' ),
				);
		} // End switch().
	}