Index: modules/file/tests/file.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/file/tests/file.test,v
retrieving revision 1.16
diff -u -p -r1.16 file.test
--- modules/file/tests/file.test	29 Jun 2010 00:57:19 -0000	1.16
+++ modules/file/tests/file.test	30 Jun 2010 13:22:13 -0000
@@ -491,8 +491,8 @@ class FileFieldValidateTestCase extends 
     $field = field_info_field($field_name);
     $instance = field_info_instance('node', $field_name, $type_name);
 
-    // Get the test file (a GIF image).
     $test_file = $this->getTestFile('image');
+    list(, $test_file_extension) = explode('.', $test_file->filename);
 
     // Disable extension checking.
     $this->updateFileField($field_name, $type_name, array('file_extensions' => ''));
@@ -513,7 +513,7 @@ class FileFieldValidateTestCase extends 
     $this->assertRaw($error_message, t('Node save failed when file uploaded with the wrong extension.'));
 
     // Enable extension checking for text and image files.
-    $this->updateFileField($field_name, $type_name, array('file_extensions' => 'txt gif'));
+    $this->updateFileField($field_name, $type_name, array('file_extensions' => "txt $test_file_extension"));
 
     // Check that the file can be uploaded with extension checking.
     $nid = $this->uploadNodeFile($test_file, $field_name, $type_name);
Index: modules/simpletest/tests/file.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/file.test,v
retrieving revision 1.57
diff -u -p -r1.57 file.test
--- modules/simpletest/tests/file.test	26 Jun 2010 19:55:47 -0000	1.57
+++ modules/simpletest/tests/file.test	30 Jun 2010 13:22:14 -0000
@@ -563,6 +563,7 @@ class FileSaveUploadTest extends FileHoo
     $this->drupalLogin($account);
 
     $this->image = current($this->drupalGetTestFiles('image'));
+    list(, $this->image_extension) = explode('.', $this->image->filename);
     $this->assertTrue(is_file($this->image->uri), t("The image file we're going to upload exists."));
 
     $this->phpfile = current($this->drupalGetTestFiles('php'));
@@ -665,8 +666,8 @@ class FileSaveUploadTest extends FileHoo
     // Reset the hook counters.
     file_test_reset();
 
-    $extensions = 'foo gif';
-    // Now tell file_save_upload() to allow ".gif".
+    $extensions = 'foo ' . $this->image_extension;
+    // Now tell file_save_upload() to allow the extension of our test image.
     $edit = array(
       'file_test_replace' => FILE_EXISTS_REPLACE,
       'files[file_test_upload]' => drupal_realpath($this->image->uri),
@@ -747,12 +748,12 @@ class FileSaveUploadTest extends FileHoo
   function testHandleFileMunge() {
     // Ensure insecure uploads are disabled for this test.
     variable_set('allow_insecure_uploads', 0);
-    $this->image = file_move($this->image, $this->image->uri . '.foo.gif');
+    $this->image = file_move($this->image, $this->image->uri . '.foo.' . $this->image_extension);
 
     // Reset the hook counters to get rid of the 'move' we just called.
     file_test_reset();
 
-    $extensions = 'gif';
+    $extensions = $this->image_extension;
     $edit = array(
       'files[file_test_upload]' => drupal_realpath($this->image->uri),
       'extensions' => $extensions,
@@ -760,7 +761,7 @@ class FileSaveUploadTest extends FileHoo
 
     $munged_filename = $this->image->filename;
     $munged_filename = substr($munged_filename, 0, strrpos($munged_filename, '.'));
-    $munged_filename .= '_.gif';
+    $munged_filename .= '_.' . $this->image_extension;
 
     $this->drupalPost('file-test/upload', $edit, t('Submit'));
     $this->assertResponse(200, t('Received a 200 response for posted test file.'));