Text wrongly appears when dragging and dropping thumbnails or images into "Notes" sidebar

[Softvision-CristinaBadescu]

[Notes]:

• Since we want to support drag and drop (Text can be wrongly inserted in the "Notes" sidebar with drag and drop #436), we should still restrict some elements from being dragged and dropped in the sidebar, as they can result in seemingly random text being pasted.

[Affected versions]:

• Firefox 57 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile with the latest "Firefox Notes" add-on v1.9.0dev installed.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to YouTube.
2. Drag a video thumbnail and drop it in the "Notes" sidebar.
3. Observe the behavior.

[Expected result]:

• Nothing happens.

[Actual result]:

• Text appears.

[Additional notes]:

• If you select the thumbnail and perform copy/paste action into the "Notes" sidebar, nothing appears.
• This issue is also reproducible with Google images or with thumbnails from Vimeo.
• This issue is also reproducible with "Notes" v1.9.0rc1.
• Here is a screen recording of the issue:
  

[vladikoff]

Not much we can do in this case. Closing as WONTFIX for now