I just checked the source and found that the extension doesn't declare read permission. 
So you will have to additionally drag File component and tick ReadPermission property.
It is working fine if I grant read permission.
Only one extension can use the library, so other extension's developers will have to remove jar from their extensions.
There is a second method too if extension developer is not active in the community.
Still? You never showed the blocks.
I assume that remark is tongue-in-cheek 
What can be done on the App Inventor side to prevent 'jar clash'? Could App Inventor host the jars?
extensions are incompatible in this case...
from the App Inventor Extensions document, chapter 3.3.2 Using external libraries
In addition to your own Java code, you can also include external libraries, by following the steps in How to Add a Component. The library jar files will be included in the aix file when you package your extension
One complication here is that the external libraries (jar files) must be unique across projects. If a project imports several extensions, then two different extensions cannot import the same jar file.
Taifun
Something needs to be done about this. There are more extension developers and more extensions every week. How unique must a jar be? In name only or also in stored contents?
Can the App Inventor side prevent this issue? A registration of jars used by extensions perhaps?
Both the events are triggered if any error occurs so what's happening with your app is likely to happen.
But, before asking a question in App Inventor Community make sure you post the blocks of AI2.
does anyone have an aia file to share by download link? just a minimal working app. I can't get mine working.thanks
what about following the 3. Usages chapter of the first post in this thread, see
Taifun
Of course. i followed 3, using my own info. I tried the other successful example. I can't get it to work.
An aia file would give me a working example to modify. thanks
It would really help if you provided a screenshot of your relevant blocks, so we can see what you are trying to do, and where the problem may be.
To get an image of your blocks, right click in the Blocks Editor and select "Download Blocks as Image". You might want to use an image editor to crop etc. if required. Then post it here in the community.
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
i used/modified your aia file for listing installed apps, and it worked great, thanks. If you could just export an aia file and email it or post it, I'm sure I could get the example working and modify it very quickly. I could then write a quick post explaining my errors. Thanks
I'm sure we can find out what is wrong in your blocks... you only have to post them... thanks...
Taifun
does anyone have working aia code to share on this? preferably a bare bones example? there have been a number people who have had problems with the posted examples, but later have gotten it to work. ....e.g.
with a simple example, i could get mine working and share my bugs. a back and forth with questions and answers and posting code is time consuming with a many wrong paths, as conversations show.
I agree.. therefore you should post a screenshot of your relevant blocks so we can continue
Taifun
if anyone has working aia code to share, I'd appreciate it, as there is on most every extension. It would lead to a quick solution about which I could then make one posting. Thanks
see the first post in the topic for an example of how to use the blocks.
Hi @jhn
If there had been designer properties or complex logics for setup/initialization of extension, then I would have surely posted the aia file.
You can find the demo blocks in first post.
Hi may I ask how can I make attached?
Get the absolute file path to the file on your device (ensuring it is readable), put that in a list, attach the list to the attachments socket in the extension block. Read posts 44,45,46 in this topic.
Hi thank you for replay, but the path i setting it doesn't sent attachment, can you help me look , thank you very much !!
