Prevent .m4v video uploads on minter

Currently the UI accepts `.m4v` files for minting which is Apple's DRM protected version of mp4. We don't support this on the backend currently so we want to prevent user's from being able to select it when choosing files in the UI.

Also we need better error messaging showing this file was not allowed from the backend.

A user told us they were able to mint a `.m4v` file on solsea, so consider supporting on the backend in the future.

No file or test is named. Start by locating the UI file-selection validation and the backend rejection or error-message path; verify that .m4v cannot be selected, backend rejection is clearly reported, and existing supported uploads still work.

Written by the indexing model from the issue text.