If you want to convert a file to a different extension, you'll need to do so using the proper application.
Renaming the file in Fieldguide alone will not properly convert your file.
In order to change the extension without causing issues, you'll need to follow these steps:
Download the file and open in the appropriate application
Go to File > Save As
In this window, select the desired extension and click Save
Once you've saved this file to your computer, you can upload it as a new version of the same file in Fieldguide.
Make sure the file name in Fieldguide has the new extension at the end
These instructions may vary depending on the type of file. Consult the documentation for your file's application for more accurate instructions.