That's odd that it's throwing that error. It's true that only image files can be attached, but you can try to place the contents of the files you're trying to send us inside a "Code" block. There's a little button on the rich text editor for inserting code. This should not try to render your file contents as HTML and let you proceed.
I'm facing same error as well when I add this configuration to Hue's hue_safety_valve.ini via CM:
* I've verified that the 'Enable HBase Thrift Server Framed Transport' is unchecked
* thrift_transport is set to 'buffered' in Hue
Is there a way to download the hbase-site.xml via CM?
Please let me know if there is any other config to update.