content field contains attached file name and node creation date. How to change this behavior so only body field go into content

The content field is a node_render of the node.
You can change this in the UI : admin/structure/types/manage/NODE_TYPE/display/search_index (if you don't have the search index field display type, you can easily add it in the settings there.)

// Build the node body.
  $build = node_view($node, 'search_index', !empty($node->language) ? $node->language : LANGUAGE_NONE);
  // Remove useless html crap out of the render.
  unset($build['#theme']);
  $text = drupal_render($build);
  $document->content = apachesolr_clean_text($text);

I suggest you figure out how to alter the search_index render and take it from there. Or overwrite the content field in the hooks that are designed to so?

I have the same issue, body field and another custom text field are merged into the content field but not available seperately. In which form/ui can this be done?

Forget about my message before. I managed to do with the hook hook_apachesolr_index_document_build_ENTITY_TYP