diff --git a/gallery_dl/downloader/http.py b/gallery_dl/downloader/http.py
index 56224626..e159f125 100644
--- a/gallery_dl/downloader/http.py
+++ b/gallery_dl/downloader/http.py
@@ -189,7 +189,7 @@ class HttpDownloader(DownloaderBase):
 
             # check filename extension against file header
             if adjust_extension and not offset and \
-                    pathfmt.extension in FILE_SIGNATURES:
+                    pathfmt.extension in SIGNATURE_CHECKS:
                 try:
                     file_header = next(
                         content if response.raw.chunked
@@ -220,7 +220,7 @@ class HttpDownloader(DownloaderBase):
                     offset += len(file_header)
                 elif offset:
                     if adjust_extension and \
-                            pathfmt.extension in FILE_SIGNATURES:
+                            pathfmt.extension in SIGNATURE_CHECKS:
                         self._adjust_extension(pathfmt, fp.read(16))
                     fp.seek(offset)
 
@@ -308,10 +308,9 @@ class HttpDownloader(DownloaderBase):
     @staticmethod
     def _adjust_extension(pathfmt, file_header):
         """Check filename extension against file header"""
-        sig = FILE_SIGNATURES[pathfmt.extension]
-        if not file_header.startswith(sig):
-            for ext, sig in FILE_SIGNATURES.items():
-                if file_header.startswith(sig):
+        if not SIGNATURE_CHECKS[pathfmt.extension](file_header):
+            for ext, check in SIGNATURE_CHECKS.items():
+                if check(file_header):
                     pathfmt.set_extension(ext)
                     return True
         return False
@@ -362,27 +361,30 @@ MIME_TYPES = {
 }
 
 # https://en.wikipedia.org/wiki/List_of_file_signatures
-FILE_SIGNATURES = {
-    "jpg" : b"\xFF\xD8\xFF",
-    "png" : b"\x89PNG\r\n\x1A\n",
-    "gif" : (b"GIF87a", b"GIF89a"),
-    "bmp" : b"BM",
-    "webp": b"RIFF",
-    "svg" : b"<?xml",
-    "ico" : b"\x00\x00\x01\x00",
-    "cur" : b"\x00\x00\x02\x00",
-    "psd" : b"8BPS",
-    "webm": b"\x1A\x45\xDF\xA3",
-    "ogg" : b"OggS",
-    "wav" : b"RIFF",
-    "mp3" : (b"\xFF\xFB", b"\xFF\xF3", b"\xFF\xF2", b"ID3"),
-    "zip" : (b"PK\x03\x04", b"PK\x05\x06", b"PK\x07\x08"),
-    "rar" : b"\x52\x61\x72\x21\x1A\x07",
-    "7z"  : b"\x37\x7A\xBC\xAF\x27\x1C",
-    "pdf" : b"%PDF-",
-    "swf" : (b"CWS", b"FWS"),
+SIGNATURE_CHECKS = {
+    "jpg" : lambda s: s[0:3] == b"\xFF\xD8\xFF",
+    "png" : lambda s: s[0:8] == b"\x89PNG\r\n\x1A\n",
+    "gif" : lambda s: s[0:6] in (b"GIF87a", b"GIF89a"),
+    "bmp" : lambda s: s[0:2] == b"BM",
+    "webp": lambda s: (s[0:4] == b"RIFF" and
+                       s[8:12] == b"WEBP"),
+    "svg" : lambda s: s[0:5] == b"<?xml",
+    "ico" : lambda s: s[0:4] == b"\x00\x00\x01\x00",
+    "cur" : lambda s: s[0:4] == b"\x00\x00\x02\x00",
+    "psd" : lambda s: s[0:4] == b"8BPS",
+    "webm": lambda s: s[0:4] == b"\x1A\x45\xDF\xA3",
+    "ogg" : lambda s: s[0:4] == b"OggS",
+    "wav" : lambda s: (s[0:4] == b"RIFF" and
+                       s[8:12] == b"WAVE"),
+    "mp3" : lambda s: (s[0:3] == b"ID3" or
+                       s[0:2] in (b"\xFF\xFB", b"\xFF\xF3", b"\xFF\xF2")),
+    "zip" : lambda s: s[0:4] in (b"PK\x03\x04", b"PK\x05\x06", b"PK\x07\x08"),
+    "rar" : lambda s: s[0:6] == b"\x52\x61\x72\x21\x1A\x07",
+    "7z"  : lambda s: s[0:6] == b"\x37\x7A\xBC\xAF\x27\x1C",
+    "pdf" : lambda s: s[0:5] == b"%PDF-",
+    "swf" : lambda s: s[0:3] in (b"CWS", b"FWS"),
     # check 'bin' files against all other file signatures
-    "bin" : b"\x00\x00\x00\x00\x00\x00\x00\x00",
+    "bin" : lambda s: False,
 }
 
 __downloader__ = HttpDownloader