feat(gatsby-source-s3): download non-image files

I'd like to be able to retrieve files other than images from S3, and access them through GraphQL.

Currently, localfile nodes are only generated in GraphQL for image files, and not e.g.: mdx, json or other data/text files.

I'm specifically interested in mdx files (which I assume could then be ingested by gatsby-transformer-mdx).

Is there a design reason for only creating local file nodes for images? If there are drawbacks to handling all files, a config option for other file extensions to accept seems like a clean solution, e.g.:
`{
resolve: 'gatsby-source-s3',
options: {
aws: {
...
},
buckets: [...],
file_extensions: ['.mdx', '.json'],
},`