fix: Fixed pocketbook extractor trying to read all files

The complete read routine would work before figuring out that it is
a file of xml mimetype. This means that it would try to read to memory
any file as the first thing, pdfs, even binaries. Of course doing
so crashed the program.

papis_extract/extractors/pocketbook.py

@@ -12,11 +12,11 @@ logger = papis.logging.get_logger(__name__)
 
 class PocketBookExtractor:
     def can_process(self, filename: Path) -> bool:
-        content = self._read_file(filename)
-        if not content:
+        if not magic.from_file(filename, mime=True) == "text/xml":
             return False
 
-        if not magic.from_buffer(content, mime=True) == "text/xml":
+        content = self._read_file(filename)
+        if not content:
             return False
 
         html = BeautifulSoup(content, features="xml")