Updated Concepts (markdown)

Concepts.md

@@ -32,6 +32,17 @@ Please not, that no thumbnails are generated for private attachments, which can
 
 Part-DB ships some preview images for various common footprints like DIP-8 and others, as internal ressources. These can be accessed/searched by typing the keyword in the URL field of a part and choosing one of the choices from the dropdown.
 
+### Preview image / attachment
+Most entities with attachments allow you to select one of the defined attachments as "Preview image". You can select an image attachment here, that previews the entity, this could be a picture of a Part, the logo of a manufacturer or supplier, the schematic symbol of a category or the image of an footprint.
+The preview image will be shown in various locations together with the entities name.
+
+Please note that as long as the picture is not secret, it should be stored on the Part-DB instance (by upload, or letting Part-DB download the file) and *not* be marked as a private attachments, so that thumbnails can be generated for the picture (which improves performance). 
+
+
+### Attachment types
+Attachment types define logical groups of attachments. For example you could define an attachment group "Datasheets" where all datasheets of Parts, Footprints, etc. belong in, "Pictures" for preview images and more.
+You can define file type restrictions, which file types and extensions are allowed for files with that attachment type.
+
 ## User System
 ### User
 Each person which should be able to use Part-DB (by logging in) is represented by an user entity, which defines things like access rights, the password, and other things. For security reasons, every person which will use Part-DB should use its own personal account with an secret password. This allows to track activity of the users via the log.