added accept attribute for img only

[codetheorem]

Issue Number

fixes #189

Describe the changes you've made

Added accept attribute so that any user can only upload image files in comment.

Test plan (OPTIONAL)

1. Go to a story.
2. click on attachment icon in comment form.

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] All new and existing tests passed.
• [x] The title of my pull request is a short description of the requested changes.

[cyntss]

In some cases, we will need to have other types of files, such as PDF or word documents if someone needs to provide full documentation. Could you update the PR to reflect these file types? also, it would be great to add some sort of information pop-up to show the users the type of files accepted