diff --git a/src/calibre/ebooks/conversion/archives.py b/src/calibre/ebooks/conversion/archives.py
new file mode 100644
index 0000000000..15d30c05e0
--- /dev/null
+++ b/src/calibre/ebooks/conversion/archives.py
@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+# License: GPLv3 Copyright: 2025, Kovid Goyal <kovid at kovidgoyal.net>
+
+import os
+import re
+
+from calibre import extract, filesystem_encoding, walk
+
+ARCHIVE_FMTS = ('zip', 'rar', 'oebzip')
+
+
+def unarchive(path, tdir):
+    extract(path, tdir)
+    files = list(walk(tdir))
+    files = [f if isinstance(f, str) else f.decode(filesystem_encoding)
+            for f in files]
+    from calibre.customize.ui import available_input_formats
+    fmts = set(available_input_formats())
+    fmts -= {'htm', 'html', 'xhtm', 'xhtml'}
+    fmts -= set(ARCHIVE_FMTS)
+
+    for ext in fmts:
+        for f in files:
+            if f.lower().endswith('.'+ext):
+                if ext in ['txt', 'rtf'] and os.stat(f).st_size < 2048:
+                    continue
+                return f, ext
+    return find_html_index(files)
+
+
+def find_html_index(files):
+    '''
+    Given a list of files, find the most likely root HTML file in the
+    list.
+    '''
+    html_pat = re.compile(r'\.(x){0,1}htm(l){0,1}$', re.IGNORECASE)
+    html_files = [f for f in files if html_pat.search(f) is not None]
+    if not html_files:
+        raise ValueError(_('Could not find an e-book inside the archive'))
+    html_files = [(f, os.stat(f).st_size) for f in html_files]
+    html_files.sort(key=lambda x: x[1])
+    html_files = [f[0] for f in html_files]
+    for q in ('toc', 'index'):
+        for f in html_files:
+            if os.path.splitext(os.path.basename(f))[0].lower() == q:
+                return f, os.path.splitext(f)[1].lower()[1:]
+    return html_files[-1], os.path.splitext(html_files[-1])[1].lower()[1:]
diff --git a/src/calibre/ebooks/conversion/plumber.py b/src/calibre/ebooks/conversion/plumber.py
index 73fba0a4d5..1866abef2e 100644
--- a/src/calibre/ebooks/conversion/plumber.py
+++ b/src/calibre/ebooks/conversion/plumber.py
@@ -5,12 +5,11 @@ __docformat__ = 'restructuredtext en'
 import json
 import os
 import pprint
-import re
 import shutil
 import sys
 from functools import partial
 
-from calibre import extract, filesystem_encoding, get_types_map, isbytestring, walk
+from calibre import filesystem_encoding, get_types_map, isbytestring
 from calibre.constants import __version__
 from calibre.customize.conversion import DummyReporter, OptionRecommendation
 from calibre.customize.ui import (
@@ -23,6 +22,7 @@ from calibre.customize.ui import (
     run_plugins_on_postprocess,
     run_plugins_on_preprocess,
 )
+from calibre.ebooks.conversion.archives import ARCHIVE_FMTS, unarchive
 from calibre.ebooks.conversion.preprocess import HTMLPreProcessor
 from calibre.ptempfile import PersistentTemporaryDirectory
 from calibre.utils.date import parse_date
@@ -74,9 +74,6 @@ class CompositeProgressReporter:
         self.global_reporter(global_frac, msg)
 
 
-ARCHIVE_FMTS = ('zip', 'rar', 'oebzip')
-
-
 class Plumber:
     '''
     The `Plumber` manages the conversion pipeline. An UI should call the methods
@@ -746,7 +743,7 @@ OptionRecommendation(name='search_replace',
         if input_fmt in ARCHIVE_FMTS:
             self.log('Processing archive...')
             tdir = PersistentTemporaryDirectory('_pl_arc')
-            self.input, input_fmt = self.unarchive(self.input, tdir)
+            self.input, input_fmt = unarchive(self.input, tdir)
             self.archive_input_tdir = tdir
         if os.access(self.input, os.R_OK):
             nfp = run_plugins_on_preprocess(self.input, input_fmt)
@@ -811,43 +808,6 @@ OptionRecommendation(name='search_replace',
             self.merge_plugin_recommendations()
 
     @classmethod
-    def unarchive(self, path, tdir):
-        extract(path, tdir)
-        files = list(walk(tdir))
-        files = [f if isinstance(f, str) else f.decode(filesystem_encoding)
-                for f in files]
-        from calibre.customize.ui import available_input_formats
-        fmts = set(available_input_formats())
-        fmts -= {'htm', 'html', 'xhtm', 'xhtml'}
-        fmts -= set(ARCHIVE_FMTS)
-
-        for ext in fmts:
-            for f in files:
-                if f.lower().endswith('.'+ext):
-                    if ext in ['txt', 'rtf'] and os.stat(f).st_size < 2048:
-                        continue
-                    return f, ext
-        return self.find_html_index(files)
-
-    @classmethod
-    def find_html_index(self, files):
-        '''
-        Given a list of files, find the most likely root HTML file in the
-        list.
-        '''
-        html_pat = re.compile(r'\.(x){0,1}htm(l){0,1}$', re.IGNORECASE)
-        html_files = [f for f in files if html_pat.search(f) is not None]
-        if not html_files:
-            raise ValueError(_('Could not find an e-book inside the archive'))
-        html_files = [(f, os.stat(f).st_size) for f in html_files]
-        html_files.sort(key=lambda x: x[1])
-        html_files = [f[0] for f in html_files]
-        for q in ('toc', 'index'):
-            for f in html_files:
-                if os.path.splitext(os.path.basename(f))[0].lower() == q:
-                    return f, os.path.splitext(f)[1].lower()[1:]
-        return html_files[-1], os.path.splitext(html_files[-1])[1].lower()[1:]
-
     def get_all_options(self):
         ans = {}
         for group in (self.input_options, self.pipeline_options,