Add a tweak in Preferences->Tweaks to exclude some images types from being treated a covers when dropped onto the Book Details panel. Fixes #1620198 [{Enhancement] Tweak to control image DnD to Book Details](https://bugs.launchpad.net/calibre/+bug/1620198)

resources/default_tweaks.py

@@ -565,3 +565,11 @@
 # numbers.
 # The value can be between 50 and 99
 content_server_thumbnail_compression_quality = 75
+
+#: Image file types to treat as ebooks when dropping onto the Book Details panel
+# Normally, if you drop any image file in a format known to calibre onto the
+# Book Details panel, it will be used to set the cover. If you want to store
+# some image types as ebooks instead, you can set this tweak.
+# Examples:
+#    cover_drop_exclude = {'tiff', 'webp'}
+cover_drop_exclude = ()

src/calibre/gui2/actions/add.py

@@ -18,6 +18,7 @@
 from calibre.gui2.dialogs.confirm_delete import confirm
 from calibre.gui2.dialogs.progress import ProgressDialog
 from calibre.ebooks import BOOK_EXTENSIONS
+from calibre.utils.config_base import tweaks
 from calibre.utils.filenames import ascii_filename
 from calibre.utils.icu import sort_key
 from calibre.gui2.actions import InterfaceAction
@@ -359,11 +360,12 @@ def files_dropped_on_book(self, event, paths, cid=None, do_confirm=True):
             cid = db.id(current_idx.row()) if cid is None else cid
         formats = []
         from calibre.gui2.dnd import image_extensions
+        image_exts = set(image_extensions()) - set(tweaks['cover_drop_exclude'])
         for path in paths:
             ext = os.path.splitext(path)[1].lower()
             if ext:
                 ext = ext[1:]
-            if ext in image_extensions():
+            if ext in image_exts:
                 pmap = QPixmap()
                 pmap.load(path)
                 if not pmap.isNull():

src/calibre/gui2/book_details.py

@@ -653,7 +653,8 @@ def dropEvent(self, event):
         event.setDropAction(Qt.CopyAction)
         md = event.mimeData()
 
-        x, y = dnd_get_image(md)
+        image_exts = set(image_extensions()) - set(tweaks['cover_drop_exclude'])
+        x, y = dnd_get_image(md, image_exts)
         if x is not None:
             # We have an image, set cover
             event.accept()
@@ -668,7 +669,7 @@ def dropEvent(self, event):
                 return
 
         # Now look for ebook files
-        urls, filenames = dnd_get_files(md, BOOK_EXTENSIONS, allow_all_extensions=True, filter_exts=image_extensions())
+        urls, filenames = dnd_get_files(md, BOOK_EXTENSIONS, allow_all_extensions=True, filter_exts=image_exts)
         if not urls:
             # Nothing found
             return