Summary

Unable to install add-on from Confluence attachment url

Steps to Reproduce

1. Install a vanilla Confluence 5.10.X instance
2. Access the instance and uploaded a valid OBR file as an attachment
3. Copy the URL to the attachment
4. Go to -> Add-Ons
5. Click on Upload add-on
6. Paste the URL we copied to the "From this URL" field
7. Click upload

Expected Results

The URL is pointing to a valid OBR file, so the add-on should be uploaded as stated in the guide Installing by file upload

Actual Results

The below exception is thrown in the atlassian-confluence.log file:

2016-08-15 18:50:17,191 ERROR [UpmAsynchronousTaskManager:thread-1] [rest.resources.install.InstallFromUriTask] download Error downloading plugin from http://localhost:PORT/confluence/download/attachments/folder/fileName.obr?api=v2
 -- url: /confluence/rest/plugins/1.0/ | referer: http://localhost:PORT/confluence/plugins/servlet/upm | userName: admin

Notes

Also tested this using a publically accessible Confluence instance using SSL and a domain. The issue is the same with other file types such as JAR. Additionally, the issue can be replicated in other applications that use UPM such as JIRA, meaning this us a UPM issue not specific to one application. This same behavior happens in the following tested versions of UPM:

2.18.2
2.18.4
2.18.5
2.19.1
2.20.0
2.21.0
2.21.3

Workaround

The workaround is to download the file in a browser and upload via the "From my computer" option:

1. Go to -> Add-Ons
2. Click on Upload add-on
3. Select the file using the "From my computer" option
4. Click upload