diff --git a/api/src/org/labkey/api/premium/DefaultAVMultipartResolver.java b/api/src/org/labkey/api/premium/DefaultAVMultipartResolver.java
index cd3b963cbf4..0899c3b8f77 100644
--- a/api/src/org/labkey/api/premium/DefaultAVMultipartResolver.java
+++ b/api/src/org/labkey/api/premium/DefaultAVMultipartResolver.java
@@ -3,8 +3,11 @@
 import jakarta.servlet.ServletException;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.Part;
+import org.apache.commons.lang3.StringUtils;
 import org.jetbrains.annotations.NotNull;
+import org.labkey.api.action.ApiUsageException;
 import org.labkey.api.util.FileUtil;
+import org.labkey.api.view.UnauthorizedException;
 import org.springframework.web.multipart.MultipartException;
 import org.springframework.web.multipart.MultipartHttpServletRequest;
 import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
@@ -22,10 +25,17 @@ public class DefaultAVMultipartResolver extends StandardServletMultipartResolver
             for (Part part : request.getParts())
             {
                 // Filter to just file uploads
-                if (part.getSubmittedFileName() != null)
+                if (!StringUtils.isEmpty(part.getSubmittedFileName()))
                 {
-                    FileUtil.checkAllowedFileName(part.getSubmittedFileName(), true);
-                    validate(part);
+                    try
+                    {
+                        FileUtil.checkAllowedFileName(part.getSubmittedFileName(), true);
+                        validate(part);
+                    }
+                    catch (IOException e)
+                    {
+                        throw new InvalidFileExtensionException(e.getMessage());
+                    }
                 }
             }
         }
@@ -40,4 +50,12 @@ protected void validate(Part part)
     {
         //do nothing by default, but give subclasses a chance to override
     }
+
+    public static class InvalidFileExtensionException extends ApiUsageException
+    {
+        public InvalidFileExtensionException(String msg)
+        {
+            super(msg);
+        }
+    }
 }
diff --git a/api/src/org/labkey/api/util/FileUtil.java b/api/src/org/labkey/api/util/FileUtil.java
index 8446e0c3535..ff7dece5515 100644
--- a/api/src/org/labkey/api/util/FileUtil.java
+++ b/api/src/org/labkey/api/util/FileUtil.java
@@ -326,7 +326,7 @@ static String isAllowedFileName(String s, boolean checkFileExtension, AppProps a
         {
             String badExtension = checkExtension(s, AppProps.getInstance());
             if (badExtension != null)
-                return "This file type [" + badExtension + "] is not allowed.";
+                return "This file type [" + badExtension + "] is not allowed. Accepted file extensions: " + AppProps.getInstance().getAllowedExtensions();
         }
         return null;
     }
@@ -1533,11 +1533,6 @@ else if (ch == '-' &&
         if (ch == ' ' || ch == '.')
             ret[lastIndex] = '_';
 
-        String result = new String(ret);
-
-        assert !AppProps.getWriteableInstance().isInvalidFilenameBlocked() || isAllowedFileName(result, true) == null :
-                "Failed to make filename safe. Original: " + name + ", transformed: " + result + ", error: " + isAllowedFileName(result, true);
-
         return new String(ret);
     }
 
diff --git a/api/src/org/labkey/api/view/MockHttpResponseWithRealPassthrough.java b/api/src/org/labkey/api/view/MockHttpResponseWithRealPassthrough.java
index ef97b11cf24..41f136bbeb8 100644
--- a/api/src/org/labkey/api/view/MockHttpResponseWithRealPassthrough.java
+++ b/api/src/org/labkey/api/view/MockHttpResponseWithRealPassthrough.java
@@ -15,6 +15,7 @@
  */
 package org.labkey.api.view;
 
+import org.jetbrains.annotations.NotNull;
 import org.labkey.api.action.ApiUsageException;
 import org.springframework.mock.web.MockHttpServletResponse;
 
@@ -50,7 +51,7 @@ public HttpServletResponse getResponse()
     }
 
     @Override
-    public PrintWriter getWriter() throws UnsupportedEncodingException
+    public @NotNull PrintWriter getWriter() throws UnsupportedEncodingException
     {
         if (_writer == null)
         {
@@ -76,7 +77,7 @@ public SizeLimitExceededException()
     /** Checks the size of the current output to make sure it hasn't exceeded the limit before appending the new content */
     private class SizeLimitingPrintWriter extends PrintWriter
     {
-        private PrintWriter _out;
+        private final PrintWriter _out;
 
         public SizeLimitingPrintWriter(PrintWriter out)
         {
@@ -100,28 +101,28 @@ private void checkSize()
         }
 
         @Override
-        public void write(char[] buf, int off, int len)
+        public void write(char @NotNull [] buf, int off, int len)
         {
             checkSize();
             _out.write(buf, off, len);
         }
 
         @Override
-        public void write(char[] buf)
+        public void write(char @NotNull [] buf)
         {
             checkSize();
             _out.write(buf);
         }
 
         @Override
-        public void write(String s, int off, int len)
+        public void write(@NotNull String s, int off, int len)
         {
             checkSize();
             _out.write(s, off, len);
         }
 
         @Override
-        public void write(String s)
+        public void write(@NotNull String s)
         {
             checkSize();
             _out.write(s);
@@ -170,7 +171,7 @@ public void print(double d)
         }
 
         @Override
-        public void print(char[] s)
+        public void print(char @NotNull [] s)
         {
             checkSize();
             _out.print(s);
@@ -240,7 +241,7 @@ public void println(double x)
         }
 
         @Override
-        public void println(char[] x)
+        public void println(char @NotNull [] x)
         {
             checkSize();
             _out.println(x);
@@ -261,28 +262,28 @@ public void println(Object x)
         }
 
         @Override
-        public PrintWriter printf(String format, Object... args)
+        public PrintWriter printf(@NotNull String format, Object... args)
         {
             checkSize();
             return _out.printf(format, args);
         }
 
         @Override
-        public PrintWriter printf(Locale l, String format, Object... args)
+        public PrintWriter printf(Locale l, @NotNull String format, Object... args)
         {
             checkSize();
             return _out.printf(l, format, args);
         }
 
         @Override
-        public PrintWriter format(String format, Object... args)
+        public PrintWriter format(@NotNull String format, Object... args)
         {
             checkSize();
             return _out.format(format, args);
         }
 
         @Override
-        public PrintWriter format(Locale l, String format, Object... args)
+        public PrintWriter format(Locale l, @NotNull String format, Object... args)
         {
             checkSize();
             return _out.format(l, format, args);