diff --git a/src/bun.js/webcore/blob.zig b/src/bun.js/webcore/blob.zig
index b8d344ece3..5a41f5f8ce 100644
--- a/src/bun.js/webcore/blob.zig
+++ b/src/bun.js/webcore/blob.zig
@@ -2458,6 +2458,9 @@ pub const Blob = struct {
                     var slicer = zig_str.toSlice(bun.default_allocator);
                     defer slicer.deinit();
                     var slice = slicer.slice();
+                    if (!strings.isAllASCII(slice)) {
+                        break :inner;
+                    }
 
                     if (globalThis.bunVM().mimeType(slice)) |mime| {
                         content_type = mime.value;