diff --git a/ChangeLog.md b/ChangeLog.md
index 1e534bf0..4f5f8bcb 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -7,6 +7,8 @@
 - Added the ability to copy a link, as Gemtext, to the clipboard
   (`gemtext_link.copy_link_as_gemtext`).
   ([#492](https://github.com/davep/rogallo/pull/492))
+- Fixed MIME type checks to ensure parameters are ignored when performing
+  the check. ([#495](https://github.com/davep/rogallo/pull/495))
 
 ## v3.1.0
 
diff --git a/src/rogallo/document.py b/src/rogallo/document.py
index 0e84e615..00d66a94 100644
--- a/src/rogallo/document.py
+++ b/src/rogallo/document.py
@@ -28,7 +28,7 @@
 
 ##############################################################################
 # Local imports.
-from .mime_checks import is_gemini_mime_type, is_gopher_mime_type
+from .mime_checks import clean_mime_type, is_gemini_mime_type, is_gopher_mime_type
 from .types import RogalloLocation
 
 
@@ -83,13 +83,10 @@ def __bool__(self) -> bool:
         """Return `True` if there is a location, `False` if not."""
         return self.location is not None
 
-    @cached_property
+    @property
     def mime_type_sans_parameters(self) -> str | None:
         """The MIME type cleaned of any parameters.."""
-        if self.mime_type is None:
-            return None
-        mime_type, _, _ = self.mime_type.partition(";")
-        return mime_type.strip().lower()
+        return clean_mime_type(self.mime_type)
 
     @property
     def is_gemtext(self) -> bool:
diff --git a/src/rogallo/mime_checks.py b/src/rogallo/mime_checks.py
index 2281c3c4..b5d131da 100644
--- a/src/rogallo/mime_checks.py
+++ b/src/rogallo/mime_checks.py
@@ -47,6 +47,23 @@ def is_gopher_mime_type(mime_type: str | None) -> bool:
     )
 
 
+##############################################################################
+def clean_mime_type(mime_type: str | None) -> str | None:
+    """The MIME type cleaned of any parameters.
+
+    Args:
+        mime_type: The MIME type to clean.
+
+    Returns:
+        The MIME type cleaned of any parameters, or `None` if the input was
+            `None`.
+    """
+    if mime_type is None:
+        return None
+    mime_type, _, _ = mime_type.partition(";")
+    return mime_type.strip().lower()
+
+
 ##############################################################################
 @cache
 def is_displayable_mime_type(mime_type: str | None) -> bool:
@@ -58,9 +75,8 @@ def is_displayable_mime_type(mime_type: str | None) -> bool:
     Returns:
         `True` if the MIME type is displayable, `False` otherwise.
     """
-    if mime_type is None:
+    if (mime_type := clean_mime_type(mime_type)) is None:
         return False
-    mime_type, _, _ = mime_type.partition(";")
     return mime_type.startswith("text/") or mime_type in {
         ItemType.MENU.mime_type,
         ItemType.INDEX_SEARCH.mime_type,
diff --git a/src/rogallo/screens/main/unsupported.py b/src/rogallo/screens/main/unsupported.py
index 34dddb5e..2ed69146 100644
--- a/src/rogallo/screens/main/unsupported.py
+++ b/src/rogallo/screens/main/unsupported.py
@@ -23,6 +23,7 @@
     save_trusted_schemes,
 )
 from ...messages import DownloadURI
+from ...mime_checks import clean_mime_type
 from ..confirm_unsupported import ConfirmUnsupportedURI
 from .local_messages import OpenUnsupportedMIMEType, OpenUnsupportedURI
 
@@ -91,17 +92,17 @@ async def maybe_open_unsupported_mime_type(
         )
         return
 
+    mime_type = clean_mime_type(message.mime_type) or "application/octet-stream"
+
     # If the MIME type isn't trusted, let's see what the user wants to
     # do about it.
     if not (
-        open_uri := (
-            message.mime_type in (trusted_mime_types := load_trusted_mime_types())
-        )
+        open_uri := (mime_type in (trusted_mime_types := load_trusted_mime_types()))
     ):
         match await owner.app.push_screen_wait(
             ConfirmUnsupportedURI(
                 str(message.location),
-                f"The MIME type '{message.mime_type}' is not supported by Rogallo. "
+                f"The MIME type '{mime_type}' is not supported by Rogallo. "
                 "Do you want to open the location in your external browser?",
                 allow_download=not isinstance(message.location, Path),
             )
@@ -110,7 +111,7 @@ async def maybe_open_unsupported_mime_type(
                 open_uri = True
             case "always":
                 open_uri = True
-                trusted_mime_types.add(message.mime_type)
+                trusted_mime_types.add(mime_type)
                 save_trusted_mime_types(trusted_mime_types)
             case "download":
                 open_uri = False