Common form for creating attachments (Follow-up from "Resolve "Add images for visualization directly after adding the attachment for devices/platforms/configurations/sites"")

The following discussion from !400 (merged) should be addressed:

• @nils.brinckmann started a discussion: (+3 comments)

  Does it make sense to put the 2 getters here (attachmentCanBeUsedAsImage and createImageHint) in a common file (as for the file endings)?

  I just wonder as we have the very same methods in all the new.vue files.