Add a route to get the item preview

This is the first available preview of an attachment wrt position. If
all attachments have a preview image, the preview of the first
attachment is returned.

modules/restapi/src/main/resources/docspell-openapi.yml

@@ -1847,6 +1847,47 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ItemProposals"
+  /sec/item/{id}/preview:
+    head:
+      tags: [ Attachment ]
+      summary: Get a preview image of an attachment file.
+      description: |
+        Checks if an image file showing a preview of the item is
+        available. If not available, a 404 is returned. The preview is
+        currently the an image of the first page of the first
+        attachment.
+      security:
+        - authTokenHeader: []
+      parameters:
+        - $ref: "#/components/parameters/id"
+      responses:
+        200:
+          description: Ok
+        404:
+          description: NotFound
+    get:
+      tags: [ Attachment ]
+      summary: Get a preview image of an attachment file.
+      description: |
+        Gets a image file showing a preview of the item. Usually it is
+        a small image of the first page of the first attachment. If
+        not available, a 404 is returned. However, if the query
+        parameter `withFallback` is `true`, a fallback preview image
+        is returned. You can also use the `HEAD` method to check for
+        existence.
+      security:
+        - authTokenHeader: []
+      parameters:
+        - $ref: "#/components/parameters/id"
+        - $ref: "#/components/parameters/withFallback"
+      responses:
+        200:
+          description: Ok
+          content:
+            application/octet-stream:
+              schema:
+                type: string
+                format: binary
 
   /sec/item/{itemId}/reprocess:
     post: