Limiting the number of file types is managed in OpenCart by limiting the number of file extension types.  The following OpenCart 1.5  tutorial will guide you in setting the allowed file extension types for your store. 

How to set the file extensions allowed for upload in OpenCart 1.5

1. Login to the Administration Dashboard as an Administrator
2. Hover over the menu bar where it says SYSTEM and then click on SETTINGS in the drop-down menu
3. Select the STORE that you wish to edit and then click on EDIT on the far right to proceed
4. Click on the TAB that says OPTION, then scroll down until you see the option labeled ALLOW UPLOAD FILE EXTENSIONS as shown below:
   
   
   
   
5. Select the box where you can type the extensions and then type in the extensions that you wish to add.  Separate multiple extensions with a comma
6. Select the blue SAVE button in the top right hand corner of the page to save the extensions that you added or removed.

It is important to limit the number of extensions of files that can be uploaded to your store.  This helps with security and prevents the upload of unnecessary files.