Merge branch 'attachment-handling' of https://github.com/thororm/PrivateBin into thororm-attachment-handling

apart from resolving conflicts:
- added missing docs
- inlined functions that were used in only one location
- updated unit test to support all previews
- fixed a regression that displayed the preview even when there was no preview and too early

i18n/oc.json

@@ -141,6 +141,8 @@
     "Cloned: '%s'": "Clonar: '%s'",
     "The cloned file '%s' was attached to this paste.": "Aqueste fichièr clonat '%s' es estat ajustat a aqueste tèxte.",
     "Attach a file": "Juntar un fichièr ",
+    "alternatively drag & drop a file or paste an image from the clipboard": "alternatively drag & drop a file or paste an image from the clipboard",
+    "File too large, to display a preview. Please download the attachment.": "File too large, to display a preview. Please download the attachment.",
     "Remove attachment": "Levar la pèca junta",
     "Your browser does not support uploading encrypted files. Please use a newer browser.":
         "Vòstre navigator es pas compatible amb lo mandadís de fichièrs chifrats. Mercés d’emplegar un navigator mai recent.",