The original? simply made the L optional.
OMG! That draws a standard rant:
[standard rant #4]
The definition of an idiot is someone who repeatedly does the same thing expecting a different result. Asking Apache to confirm the existence of ANY module with an <IfModule> ... </IfModule> wrapper is the same thing in the webmaster world. DON'T BE AN IDIOT! If you don't know whether a module is enabled, run the test ONCE then REMOVE the wrapper as it is EXTREMELY wasteful of Apache's resources (and should NEVER be allowed on a shared server).[/standard rant #4]
Okay, take heed and please don't abuse your server with <IfModule> wrappers. They're only there to protect the canned app developers from whiners who have no clue what there server is capable of.
From that I can see that you're on an Apache 2.x server (no leading / after the start anchor), that you don't know to escape the dot character in regex, that you're redirecting to mobile/event_uupload.php and have no clue what the No Case flag is used for. I would have used:
RewriteRule ^event_upload.html$ /mobile/event_upload.php [NC,L]
Question: Your statement "event_upload.php is in an actual directory named mobile and I need to change it to event_upload.html" is not represented in this code. What are you trying to do, change event_upload.html to event_upload.php but serve mobile/event_upload.php? If so, you'll need to simply change the file extension with an R=301 (so event.php is seen) then use another RewriteRule to add the mobile/ subdirectory with a simple Last flag.
RewriteRule ^event_upload\.html$ /mobile/event_upload.php [R=301,L]
# for testing, the R=301 will display the redirection for you. In the operational world,
# it would tell SE's to change their database to contain the new link, not the old.
# IF you're satisfied that it works and want to hide the redirection, simply remove the R=301, from the flags.
RewriteRule ^event_upload\.html$ mobile/event_upload.php [L]
You might benefit from reading the mod_rewrite tutorial linked in my signature as it contains explanations and sample code. It's helped may members and should help you, too.