Allow file source plugint to filter by mime type

The core migration sources are designed to support the core migration UI, which itself is designed to migrate all content/users from a d7 site. There is no need to limit files by mimetype, as they are all migrated at once in the core file migrations. You will want to have a custom source plugin to support your custom migration here, but it is not appropriate to include custom migration sources in Drupal core. The migration system is designed to be extensible to allow you to create targeted source plugins like this as needed.

Note: we do have an issue #3069776: SQL source plugins: allow defining conditions and join in migration yml → to allow arbitrary sql filtering on ANY plugin extending SqlBase. This is something that we would make use of in core to reduce the number of source plugins needed. It would also be a path to solving your issue. In retrospect, I'm going to close this as a duplicate of that one.