DATAMONGO-1806 - Polishing.

Remove unused import, trailing whitespaces and update Javadoc.

Original Pull Request: #506

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/gridfs/GridFsResource.java

@@ -23,7 +23,6 @@ import org.springframework.core.io.InputStreamResource;
 import org.springframework.core.io.Resource;
 
 import com.mongodb.client.gridfs.model.GridFSFile;
-import com.mongodb.gridfs.GridFSDBFile;
 
 /**
  * {@link GridFSFile} based {@link Resource} implementation.
@@ -89,7 +88,7 @@ public class GridFsResource extends InputStreamResource {
 	/**
 	 * Returns the {@link Resource}'s id.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Object getId() {
 		return file.getId();
@@ -98,7 +97,9 @@ public class GridFsResource extends InputStreamResource {
 	/**
 	 * Returns the {@link Resource}'s content type.
 	 *
-	 * @return
+	 * @return never {@literal null}.
+	 * @throws com.mongodb.MongoGridFSException in case no content type declared on {@link GridFSFile#getMetadata()} nor
+	 *           provided via {@link GridFSFile#getContentType()}.
 	 */
 	@SuppressWarnings("deprecation")
 	public String getContentType() {