Uploading a PDF with preview field shows "Error displaying your image" message

[txau]

This is a small nuance due to a race condition. Add a PDF to a template with a preview field, while the thumbnail is created, the entity is saved and it displays the error "Error displaying your image".

Not critical, but very misleading for users. It almost reads as if the PDF was not uploaded.

[aphilop]

@RafaPolit to discuss with the backend team potential ways to mark entities as "in-between" states.