[Accessibility] Accessible name missing for folder/file icons

[eileencho]

Description: The folder/file icons in the Notebook directory do not have accessible names. The icons contain important information that is not conveyed to screen readers. Screen reader users trying to navigate the directory have no information on whether the current item is a folder or a file.

Reproduce:

1. Open Notebook dashboard to see the file directory
2. Download and start screen reader (such as Voiceover which is already available on Macs, NVDA on Windows, Orca for Linux)
3. Navigate with screen reader to the file directory
4. When selecting an item from file directory, note (by listening or reading the screen reader output) that the folder/file icon is skipped and each item declares its name (folder or file name) and role (link), but there is no information about whether the item is a folder or a file

Suggestions:

• Make the folder/file icons visible to screen readers, and include alt text for the icons
• Include information conveyed by the icon with the name of each item

Example from NYU Accessibility Audit Supplement
WCAG 2.0 Guidelines Section 4.1.2
NYU Full Accessibility Audit done with @VickySteeves