Lỗi mở file Excel trong trình duyệt "The file format differs from the format that the file name extension specifies"

Khắc phục :
Use a Group Policy setting

1. Download the 2007 Office system Administrative Template files (ADM, ADMX, ADML) and Office Customization Tool from the following Microsoft Download Center Web site:
   http://www.microsoft.com/downloads/thankyou.aspx?familyId=92d8519a-e143-4aee-8f7a-e4bbaeba13e7
2. Extract the administrative template files to a folder. To do this, follow these steps:
   1. Double-click the AdminTemplates.exe file.
   2. Accept the Microsoft Software License Terms, and then click Continue.
   3. Select a folder in which to save the extracted files, and then click OK.
   4. Click OK after the files have been extracted successfully.
3. Open the Group Policy Object Editor.
   
   For more information about how to open the Group Policy Object Editor, visit the following Microsoft TechNet Web site:
   http://technet2.microsoft.com/windowsserver/en/library/03ec122f-fc65-496e-ad0d-4fd22a96a4bb1033.mspx
4. Expand User Configuration.
5. Right-click Administrative Templates, and then click Add/Remove Templates.
6. In the Add/Remove Templates dialog box, click Add.
7. Browse to the folder that contains the Administrative Templates that you extracted in step 2c.
8. Click the appropriate Excel .adm file, and then click Open. The .adm file is located in the following folder to which you extracted the Administrative Templates in step 2c: 
   \ADM\languageID
   Note The English language administrative template files are located in the EN-US languageID folder.
9. Click Close to close the Add/Remove Templates dialog box.
10. Expand Administrative Templates, expand Microsoft Office Excel <version number>, expand Excel Options, and then click Security.
11. On the Extended tab, double-click Force file extension to match file type under Setting.
12. On the Setting tab, click Enabled, click one of the following in the Force file extension to match file type drop-down list, and then click OK:
    • Allow different
    • Allow different, but warn
    • Always match file type
13. Close the Group Policy Object Editor.

Use Registry Editor

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

1. Exit Excel 2007.
2. Start Registry Editor.
   • In Windows Vista, click Start
     
     , type regedit in the Start Search box, and then press ENTER.
     
     
     
     If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
   • In Windows XP, click Start, click Run, type regedit, and then click OK.
3. Locate and then click the following registry subkey:
   HKEY_CURRENT_USER\Software\Microsoft\Office\<version number>\Excel\Security
   
   Office 2003 -> 11.0
   Office 2007 -> 12.0
   Office 2010 -> 14.0
   Office 2013 -> 15.0
   
    
4. On the Edit menu, point to New, and then click DWORD Value.
5. Type ExtensionHardening, and then press ENTER.
6. Right-click ExtensionHardening, and then click Modify.
7. In the Value data box, type the value data, and then click OK.
   
   The following list contains the value data settings that are appropriate for the ExtensionHardening setting:
   • 0: Do not check the file name extension and the file type, and bypass the function of the warning message.
   • 1: Check the file name extension and the file type. If they do not match, display the warning message.
   • 2: Check the file name extension and the file type. If they do not match, do not open the file.
   Note The default value data is 1. When the value data is set to 1, the behavior becomes the same as when no registry value is set. When the value data is set to 0, the file name extension and the file content are not checked in all situations. We do not recommend bypassing this function.
8. On the File menu, click Exit to exit Registry Editor.

Nguồn : http://support.microsoft.com/kb/948615