This plugin will allow you to attach a single file to multiple posts, but will also will allow you to detach any file.
<ul>
<li>Support http://xparkmedia.com/plugins/file-un-attach/</li>
<li>Languages http://xparkmedia.com/plugins/file-un-attach/#languages</li>
</ul>

File Un-Attach

This plugin will allow you to attach a single file to multiple posts, but will also will allow you to detach any file.

laurencebahiirwa

WordPress allows to attach (or assign) images and other attachments organized within the Media Library to posts and pages.
A file will automatically be attached to a particular post when uploaded using the Add media button in that post. There are also “Attach” links in the Media Library to manually attach files to posts. Once attached there is no way to unattach or re-attach images, pdfs or any attachment in WordPress core.
This plugin fills this gap by providing two additional links next to “Attach” in the Media Library:
<ul>
<li>Re-Attach: Allows user to choose a new post to attach the media file.</li>
<li>Unattach: Totally removes an existing relation between an attachment and a post.</li>
</ul>
<h3>Bulk Actions</h3>
The plugin also provides bulk actions for unattaching and re-attaching multiple files at once! Just mark the files you want to change, select your desired action from the Bulk Actions dropdown and hit Apply.
<h3>Note</h3>
<a href="https://stackoverflow.com/questions/45690582/image-post-attachments-not-specific-to-post/45705413#45705413" rel="nofollow ugc">My initial issue came from here</a>. Thanks to <a href="https://wordpress.org/plugins/unattach-and-re-attach-attachments//" rel="ugc">Unattach and Re-attach Media Attachments</a> which inspired this plugin. Just adding upgrades of the code to the latter so it is safer for use and have fixed some bugs.

Upgrade for Unattach and Re-attach Media Attachments

Allows to unattach and reattach images and other attachments from within the media library page.

File Un-Attach vs Upgrade for Unattach and Re-attach Media Attachments