Media is not showing on post page

You can either choose a theme that already shows full posts on Categories by default, or you can create a child theme:

Create a new folder under /wp-content/themes/ – the name is up to you, but for simplicity’s sake, you may just want to call it themename-child. So if you’re using the “twentysixteen” theme, call your folder twentysixteen-child.
Create a file called style.css in your folder. It will just need a couple of comments so WP recognizes your child theme, no actual styles are needed. You can name it anything you like (again, I suggest calling it a child so it’s obvious) – the only requirement is “template” has to be the parent theme’s folder. So for “twentysixteen”:

/* Theme Name: WPSE custom child theme Template: twentysixteen */

Look through the theme files to find the archive. Most themes have a file called category.php but if that doesn’t exist you may need archive.php. You are looking for a call to the_excerpt() which, if it is not in category.php or archive.php, may be in a template part such as archive-content.php – that would be called as get_template_part() inside of archive.php or content.php. Once you identify which one controls your categories – which you can find by replacing the_excerpt() with the_content() temporarily – copy that whole file into your child theme. Then, wherever inside that file there is a call to the_excerpt() just replace that with the_content() to display the full content instead of an excerpt. Make sure to change the parent theme file back – you don’t want to edit parent theme files directly, as when they update, your changes are overwritten. By making all of your changes through a child theme, your changes will remain in effect.