diff --git a/netkan/netkan/metadata.py b/netkan/netkan/metadata.py
index a64368c..f9b1e5b 100644
--- a/netkan/netkan/metadata.py
+++ b/netkan/netkan/metadata.py
@@ -5,6 +5,7 @@
 from hashlib import sha1
 import uuid
 import urllib.parse
+from string import Template
 from typing import Optional, List, Tuple, Union, Any, Dict, TYPE_CHECKING
 from ruamel.yaml import YAML
 import dateutil.parser
@@ -320,6 +321,7 @@ def __str__(self) -> str:
     ISODATETIME_PROPERTIES = [
         'release_date'
     ]
+    MIRROR_FILENAME_TEMPLATE = Template('$prefix-$identifier-$version.$extension')
 
     def __init__(self, filename: Optional[Union[str, Path]] = None, contents: Optional[str] = None) -> None:
         if filename:
@@ -438,7 +440,11 @@ def redistributable(self) -> bool:
     def mirror_filename(self, with_epoch: bool = True) -> Optional[str]:
         if 'download_hash' not in self._raw:
             return None
-        return f'{self.download_hash["sha1"][0:8]}-{self.identifier}-{self._format_version(with_epoch)}.{Ckan.MIME_TO_EXTENSION[self.download_content_type]}'
+        return self.MIRROR_FILENAME_TEMPLATE.safe_substitute(
+            prefix=self._mirror_prefix(),
+            identifier=self.identifier,
+            version=self._format_version(with_epoch),
+            extension=Ckan.MIME_TO_EXTENSION[self.download_content_type])
 
     def mirror_download(self, with_epoch: bool = True) -> Optional[str]:
         filename = self.mirror_filename(with_epoch)
@@ -450,6 +456,12 @@ def mirror_item(self, with_epoch: bool = True) -> str:
         return self._ia_bucket_sanitize(
             f'{self.identifier}-{self._format_version(with_epoch)}')
 
+    def _mirror_prefix(self) -> str:
+        return (self.download_hash['sha1']
+                if 'sha1' in self.download_hash
+                else self.download_hash['sha256']
+               )[0:8]
+
     # InternetArchive says:
     # Bucket names should be valid archive identifiers;
     # try someting matching this regular expression:
diff --git a/netkan/netkan/mirrorer.py b/netkan/netkan/mirrorer.py
index 975c5b0..7ff51fe 100644
--- a/netkan/netkan/mirrorer.py
+++ b/netkan/netkan/mirrorer.py
@@ -124,8 +124,22 @@ def mirrored(self, iarchive: internetarchive.session.ArchiveSession) -> bool:
             return False
         if not item.exists:
             return False
-        sha1 = self.download_hash['sha1'].lower()
-        return any(file['sha1'].lower() == sha1 for file in item.files if 'sha1' in file)
+        sha1 = self._sha1()
+        if sha1 is None:
+            return False
+        return any(file['sha1'].lower() == sha1
+                   for file in item.files
+                   if 'sha1' in file)
+
+    def _sha1(self) -> Optional[str]:
+        if 'sha1' in self.download_hash:
+            # Use hash from metadata if set
+            return self.download_hash['sha1'].lower()
+        dl_io = self.open_download()
+        if dl_io is not None:
+            # Calculate hash from file if found
+            return self.large_file_sha1(dl_io)
+        return None
 
     def license_urls(self) -> List[str]:
         return [self.LICENSE_URLS[lic]
@@ -158,6 +172,13 @@ def large_file_sha256(file: BinaryIO, block_size: int = 8192) -> str:
             sha.update(block)
         return sha.hexdigest().upper()
 
+    @staticmethod
+    def large_file_sha1(file: BinaryIO, block_size: int = 8192) -> str:
+        sha = hashlib.sha1()
+        for block in iter(lambda: file.read(block_size), b''):
+            sha.update(block)
+        return sha.hexdigest().upper()
+
     def open_if_hash_match(self, path: Path) -> Optional[BinaryIO]:
         """Check whether the file located at the given path matches our sha256.