When we clock upload, it only allows one file upload at a time. It wastes a lot of time if we upload many files. Is there a quicker way to upload many files at a time?
The web component is asynchronous so will only run one process at a time. You could try with several web components but app Inventor will only run one process at a time!
You could always try Taifun’s zip extension to unzip a zip with several files.
are you talking about the media section in the designer to upload assets into your project?
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
I upload image files
... to your project in the Designer or do you mean uploading files from the Internet to the external storage?
upload image files from my computer to the app server
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So you mean importing files to the assets of your project (aia), see here:
How to add / delete multiple files (images) to the assets (EDIT: AIA max 30MB) with 7-Zip or winRAR:
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see also tip 2 for correct image size
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by Taifun.
Do you have to download your app before you can access this "assets" folder?
I am trying to upload a large gif into the assets, and it is giving me an error that reads: Server error please try again later. I think the reason for the error is that the file is too big (it is larger than 5 MB) but when I looked on my computer in the AppInventor folder, I do not see the assets folder.
https://community.appinventor.mit.edu/t/video-player-file-size-limit/30386/4
there is a 5MB limit to what you can load into Assets
That's good, you should not be able to see it 
It is not a folder as such, which is why you can't write to it at runtime.
Yes.
- Export / download the project (aia) to your computer
- Open the aia with WinRAR / 7Zip
- Open assets
- Drag the gif (max. ~25MB) file into it
- Import the new aia in AI2.