provide mimeType instead of fileName

I have an application where I'm storing blobs and not storing the filename. I do store a: `{ title, mimeType, size }`
At the moment I'm using `render-media` by faking a filename, then internally `render-media` guesses a mimeType from my extension. This feels slightly painful when I can see the API could be extended to just allow mimeType to be passed in directly?

Would this be a welcome addition I'm happy to open a PR.