This is UI side unreadable device support.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 6609 matching lines @@
             "description": "Disk volume mount point path. The value corresponds to its Entry.fullPath in File API."
           },
           "sourceUrl": {
             "type": "string",
             "description": "The path to the mounted devicem, url of the mounted archive file or network resource."
           },
           "mountType": {
             "type": "string",
             "enum": ["device", "file", "network"],
             "description": "Type of the mount."
+          },
+          "mountCondition": {
+            "type": "string",
+            "description": "Additional data about mount, for example, that the filesystem is not supported."
           }
         }
       },
       {
         "id": "VolumeMetadata",
         "type": "object",
         "description": "Mounted disk volume metadata.",
         "properties": {
           "mountPath": {
             "type": "string",
@@ skipping 1681 matching lines @@
             "type": "function",
             "description": "Called when the browser's passwords have been cleared.",
             "optional": true,
             "parameters": []
           }
         ]
       }
     ]
   }
 ]