Consider if webm is a safe file format

Currently we convert all files to mp4 if they are neither a mp4 nor a wav. In #328 webm is added to the supported file formats, which brings in support for vp8. However vp8 is not supported by mp4, so this conversion breaks. We should consider if we can support webm as a native format as well. Things to check:

- Does **accurate** seeking work?
- Are all webm-formats supported by chrome?

Review the file-conversion behavior described in the issue and the WebM support added in #328. Check whether accurate seeking works with WebM and whether the WebM formats involved are supported by Chrome. Done means determining whether WebM can remain native without breaking supported formats or seeking.

Written by the indexing model from the issue text.