Manual:$wgIllegalFileChars

Details
This is a regex character class (i.e. a list of characters in a format suitable for a regular expression) that you want MediaWiki to convert to dashes ("-") despite being in the list of allowable title characters.

By default, file names can use the same characters as titles (see ), with the exception of those characters listed in this variable, the default being the colon, slash and backslash. All invalid characters in file names, including those listed in this variable, will automatically be converted to dashes ("-").

The default will not allow namespaces to be part of file names (example: File:Project:filename.txt). While this is generally desirable, it interferes with any extension that uses namespace indexing or protection. To keep the colon from being converted to a dash, remove the colon from the value of this variable.