Fix getMediaType logic

[btzr-io]

Changes

• Improve media type detection of files with a contentType similar to application/something

Example

Most files don't follow a standar mime-type.
A simple .pdf file can contain a vendor-specific mime-type:

 PDF -> application/x-pdf, application/pdf, application/octet-stream

Usage

Before:

  const mediaType =
    (fileInfo && Lbry.getMediaType(null, fileInfo.file_name)) || Lbry.getMediaType(contentType);

After:

  const extname = path.extname(fileName) || mime.getExtension(contenType);
  const mediaType = Lbry.getMediaType(contentType, extname);

Required by:
lbryio/lbry-desktop#1576
lbryio/lbry-desktop#1558