Boot Camp Assistant not accepting Windows 10 .iso file

For me the solution was just to Unmount the windows 10 iso.

If it's not working, I found another solution, you have to edit the info.plist of the Boot Camp Assistant's package content:

1. Open a Finder window.

2. Navigate to Applications>Utilities

3. Control Click or Right Click on Boot Camp Assistant and select "Show Package >Contents"

4. Open the Contents folder

5. Drag the file "info.plist" to your desktop (I also recommend saving it in >another place as well in case you want to revert to the original)

6. Open info.plist on your desktop with a text editor

7. Find the field labeled as below:
   <key>32BitSupportedModels</key> <array> <string>MacBook7,1</string> <string>MacBookAir5,2</string> <string>MacBookPro10,1</string> <string>MacPro5,1</string> <string>Macmini5,3</string> <string>iMac12,2</string> </array>

8. Time to find what your Mac is. Click the Apple in the top left hand corner and select About This Mac

   • Click More Info...
   • Click System Report...
   • Under Hardware Overview, look for Model Identifier and copy it

9. With your Mac's Identifier copied, go back to your open info.plist file create a string at the TOP of the entries. So if your Mac identifier was "iMac13,2" then it would look like this:
   <key>32BitSupportedModels</key> <array> <string>iMac13,2</string> <string>MacBook7,1</string> <string>MacBookAir5,2</string> <string>MacBookPro10,1</string> <string>MacPro5,1</string> <string>Macmini5,3</string> <string>iMac12,2</string> </array>

10. Save and close info.plist. Drag info.plist back into the Contents folder and select replace. (You may have to authenticate and enter your admin password).

11. Start Bootcamp and it should allow you to use your Disk/USB with both versions. I wouldn't recommend installing the x86 (32 bit) version of Windows 7, but this will allow you to get to the point of the installation to select x64 (64 bit) if you have an ISO file with both 32 bit and 64 bit combined. Hope this helps.

https://discussions.apple.com/thread/4747024

Caution, you might be resign the package:

4. Open your terminal, use the following command sudo codesign -fs - /Applications/Utilities/Boot\ Camp\ Assistant.app

https://discussions.apple.com/thread/5479879

I had the same issue and this worked for me:

• (close Boot Camp if it's running)
• Create backup of info.plist (inside package contents of Boot Camp Assistant)
• In info.plist scroll down to section Win7OnlyModels
• Delete the model ID "closest to yours" (in my case, I have a MacBookPro5,3 and the model ID in the list was MacBookPro5,5)
• Replace old info.plist file
• Run Boot Camp Assistant

Hope this works!