Board Thread:News and Announcements/@comment-25565811-20170310114851/@comment-27219153-20170311043249

General rule of thumb for naming images:
 * Is it used in a crafting grid?
 * If yes, then:
 * Is there a wiki page for that specific block/item?
 * If yes, then name it the same as the title of the wiki page.
 * If no, then name it the same as its in game name.
 * If no, then:
 * Do similar images already exist?
 * If yes, follow the naming style of those images.
 * If no, name it virtually whatever you want. Just try to keep it descriptive of the image.

So to answer your question, Mithril Ingot.png is correct, because the page Mithril Ingot is named that.