Ignore non audio files under samples

The samples folder may contain non-audio files under the instrument. For example, pictures of the instrument, tutorials, articles, urls and notes etc. It would be better to ignore such files.

### Example

#### Justification
The picture above lists non audio files that are showed under samples. Clicking on these files have no effect. But it would be nice to ignore them here. Or let us say, build the list of samples only with the supported audio files.

Locate the code that builds the samples list for an instrument and inspect how supported audio files are identified. Confirm the behavior with sample folders containing pictures, tutorials, URLs, notes, and audio files; done means non-audio files are omitted while supported audio remains listed.

Written by the indexing model from the issue text.