Question: **Scenario:** In an e-commerce system, a helpdesk user should be able to "impersonate" or "log on as" a specific customer (from the web-based administration site) so that he can support the customer first-hand - eg. when the Customer is on the phone.

Additional clarification: There is no need to keep track of the original authentication context. We simply need to provide a "Log on as" button that will log on as the customer without the helpdesk user needing to know the password.

Any ideas how this scenario could be implemented using regular ASP.NET Forms Authentication?

Answer: 
Save the information about file to MySQL, and save the last time the image was viewed - can be done via some script that would be altered everytime the image is being used.. and check the database for images not used for 30 days, delete them..
I'm not sure what does "temporary upload" mean in your app. The file is either uploaded or not, and under the ownership of a user. If a user doesn't want to do anything at the moment, you have no other choice but to leave the file where it is.

What you *can* do is put a warning somewhere on your image management page about unused images, but removing them yourself seems like a bad practice (at least from the user perspective).