Attachment Manager
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### Description
Funny things - we're using GitLab CE as our internal repository for local projects and the issues with uploaded files have never been issues, because we have shell access. :smiley:
But today I came across this problem on my Gitlab.com account. Basically, I'm concerning about the following aspects:
1. There is no way to restore a direct link to attached file if the page was accidentally refreshed or closed. You have to re-upload the file, thus creating unused duplicates.
2. There is no way to track dead images that are no longer needed/used.
3. There is no way to delete attached files if some of them were uploaded by mistake.
This applies both to wiki pages and issues.
### Proposal
As I can see them at this moment:
* **First approach:** a small attachment manager (e.g. when you can see all the attached files below the the text area).
* **Second approach:** Add a new section in Admin panel (something like Media in WordPress) that will show all attachments and where they are attached (or not)
* **Alternative approach:** make `uploads` folder (especially for Gitlab.com projects) cloneable using `git clone` or downloadable via UI
### Related issues
https://gitlab.com/gitlab-org/gitlab-foss/issues/12656
https://gitlab.com/gitlab-org/gitlab-foss/issues/14318
https://gitlab.com/gitlab-org/gitlab-foss/issues/23071 (Slightly related)
https://gitlab.com/gitlab-org/gitlab-foss/issues/21818 (Slightly related)
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