diff --git a/dot_config/mimeapps.list.tmpl b/dot_config/mimeapps.list.tmpl
deleted file mode 100644
index 6519478..0000000
--- a/dot_config/mimeapps.list.tmpl
+++ /dev/null
@@ -1,27 +0,0 @@
-[Added Associations]
-application/json=nvim.desktop;
-application/pdf=firefox.desktop;libreoffice-draw.desktop;chromium-browser.desktop;org.inkscape.Inkscape.desktop;krita_pdf.desktop;display-im7.q16.desktop;
-application/x-docbook+xml=nvim.desktop;
-application/x-yaml=nvim.desktop;
-text/csv=libreoffice-calc.desktop;krita_csv.desktop;nvim.desktop;vim.desktop;libreoffice-writer.desktop;okularApplication_txt.desktop;
-text/markdown=nvim.desktop;
-text/plain=wine-Programs-Kvaser Database Editor 3-Kvaser Database Editor 3.desktop;nvim.desktop;libreoffice-writer.desktop;vim.desktop;
-text/x-cmake=nvim.desktop;
-x-scheme-handler/geo=openstreetmap-geo-handler.desktop;
-x-scheme-handler/http=firefox.desktop;
-x-scheme-handler/https=firefox.desktop;
-
-[Default Applications]
-application/json=nvim.desktop;
-application/pdf=firefox.desktop;
-application/x-docbook+xml=nvim.desktop;
-application/x-yaml=nvim.desktop;
-text/csv=libreoffice-calc.desktop;
-text/html=firefox.desktop
-text/markdown=nvim.desktop;
-text/plain=wine-Programs-Kvaser Database Editor 3-Kvaser Database Editor 3.desktop;
-text/x-cmake=nvim.desktop;
-x-scheme-handler/geo=openstreetmap-geo-handler.desktop;
-x-scheme-handler/http=firefox.desktop;
-x-scheme-handler/https=firefox.desktop;
-x-scheme-handler/opencode=ai.opencode.desktop.desktop
diff --git a/dot_config/modify_mimeapps.list.tmpl b/dot_config/modify_mimeapps.list.tmpl
new file mode 100644
index 0000000..5a9a60a
--- /dev/null
+++ b/dot_config/modify_mimeapps.list.tmpl
@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+{{ $browser := "firefox.desktop" }}{{ if hasPrefix "UGC" .chezmoi.hostname }}{{ $browser = "default-browser.desktop" }}{{ end }}
+# chezmoi modify_ script: merge a curated set of default associations into the
+# machine's existing mimeapps.list (read from stdin) instead of replacing the
+# whole file, so each machine keeps its own entries.
+import sys
+
+BROWSER = "{{ $browser }}"
+SECTION = "[Default Applications]"
+
+# Associations enforced on every machine, in [Default Applications].
+ENFORCED = {
+    "application/json": "nvim.desktop",
+    "application/x-docbook+xml": "nvim.desktop",
+    "application/x-yaml": "nvim.desktop",
+    "text/markdown": "nvim.desktop",
+    "text/x-cmake": "nvim.desktop",
+    "text/html": BROWSER,
+    "x-scheme-handler/http": BROWSER,
+    "x-scheme-handler/https": BROWSER,
+}
+
+
+def main():
+    lines = sys.stdin.read().splitlines()
+
+    # Split into sections, preserving header lines and body order. The first
+    # (headerless) section holds any preamble before the first [Header].
+    sections = [[None, []]]
+    for line in lines:
+        stripped = line.strip()
+        if stripped.startswith("[") and stripped.endswith("]"):
+            sections.append([stripped, []])
+        else:
+            sections[-1][1].append(line)
+
+    target = next((s for s in sections if s[0] == SECTION), None)
+    if target is None:
+        target = [SECTION, []]
+        sections.append(target)
+
+    remaining = dict(ENFORCED)
+    body = []
+    for line in target[1]:
+        key = line.split("=", 1)[0].strip() if "=" in line else None
+        if key in remaining:
+            body.append("{}={};".format(key, remaining.pop(key)))
+        else:
+            body.append(line)
+
+    # Append any still-missing enforced keys before trailing blank lines.
+    trailing = []
+    while body and body[-1].strip() == "":
+        trailing.insert(0, body.pop())
+    for key, val in ENFORCED.items():
+        if key in remaining:
+            body.append("{}={};".format(key, val))
+    body.extend(trailing)
+    target[1] = body
+
+    out = []
+    for header, section_body in sections:
+        if header is not None:
+            out.append(header)
+        out.extend(section_body)
+    sys.stdout.write("\n".join(out) + "\n")
+
+
+main()