Attachment URL method (#5584)

* Attachment fully-qualified-url

Adds method to generate a fully-qualified URL for an attachment

* Construct absolute URL including host information

InvenTree/InvenTree/models.py

@@ -440,7 +440,8 @@ class InvenTreeAttachment(models.Model):
     An attachment can be either an uploaded file, or an external URL
 
     Attributes:
-        attachment: File
+        attachment: Upload file
+        link: External URL
         comment: String descriptor for the attachment
         user: User associated with file upload
         upload_date: Date the file was uploaded
@@ -564,6 +565,22 @@ class InvenTreeAttachment(models.Model):
         except Exception:
             raise ValidationError(_("Error renaming file"))
 
+    def fully_qualified_url(self):
+        """Return a 'fully qualified' URL for this attachment.
+
+        - If the attachment is a link to an external resource, return the link
+        - If the attachment is an uploaded file, return the fully qualified media URL
+        """
+
+        if self.link:
+            return self.link
+
+        if self.attachment:
+            media_url = InvenTree.helpers.getMediaUrl(self.attachment.url)
+            return InvenTree.helpers_model.construct_absolute_url(media_url)
+
+        return ''
+
 
 class InvenTreeTree(MPTTModel):
     """Provides an abstracted self-referencing tree model for data categories.