Currently there is a known issue opening the files attached to messages in the community.
They can be downloaded fine, but cannot be displayed with the built-in viewer.
We've reported to the community platform vendor and will update once this issue is fixed.
Note that many of you attach graphic files to post instead of including them inline.
Adding images inline makes it fair easier for fellow community members to help you, particularly in light of this issue.
To add images inline with your message, click on the camera icon above the text entry field in the message editor:

You will see a dialog similar to:

You can drag/drop photos as the screen suggests or click on the Choose Files button.
If the image is hosted elsewhere, you can add the direct URL for the image instead by clicking on URL on the top of the dialog.

There's also the hidden trick of pasting images from the clipboard, which can be done by clicking on the whitespace to the right of the grey box first.
Note that pasting an image directly in a message will result in an error.
Upload the files as appropriate, adjust formatting/size as needed, and click the Done button lower right.
This message was created with the above steps.