When importing data, Socrata will automatically detect if your file can be parsed and uploaded in a tabular format. In the case that the file is non-parsable, it will be uploaded as a "File and Documents" asset that can only be previewed and downloaded by a user.
If you think your data should be parsed as tabular data and it is not, it is because your file does not have an extension listed in the supported 'parseable file formats' on the upload page. Currently, the supported parseable file formats are: .csv, .tsv, .xls, .xlsx, .zip (shapefile), .json (GeoJSON only), .geojson, .kml, .kmz.
As listed above .json files are only parseable as geojson files, standard .json files will not be parsed during import.