Improve attachment handling for published data

[stephan271]

Issue Name

ImproveAttachmentHandling

Current Behaviour

• When no attachment exists in the linked datasets a default image is added.

• It is not possible to upload attachment images for published data

Expected Behaviour

• There should be no SciCat default image if no attachments exists
• It should be possible to upload image(s?) at creation time for published data which are used as attachments

[stephan271]

This is still an issue. In addition adding the image later in the edit step causes a API error code 502 in the resync API endpoint