Is there a plugin that will auto add tags using the filename of the uploaded image? For example if the filename that I will attach to the post is named blue_little_flowers.jpg it will extract blue,little,flowers and them as tags.
I've never seen one, but it doesn't sound too difficult to build - you'd just need to take the filename, run a preg_replace on it to strip out the spaces, %20s, underscores, hyphens and stuff, then create post_meta for the attachment.
You can do it with my plugin: https://wordpress.org/plugins/automatic-post-tagger/ However, regular expressions aren't supported yet so you'll have to either modify it or manually specify all tags you want to add with appropriate related words, for example: "blue" => "blue_,_blue"