"Invalid filename" warning lists a character that's not in my filename

I'm trying to attach a file named Graphs - Tree Mode - atlassian25.private - Disk Space - /export/downloads.png to EACJ and I get this earning message:

Graphs - Tree Mode - atlassian25.private - Disk Space - :export:downloads.png contains the invalid character ':'. Please rename the file and try again.

My file doesn't contain the character ':' anywhere, so this message is wrong. I can guess that one of the remaining characters is bad, but I don't know which one.