Not that one should go around creating files like #Test_page.txt, but if one does and then tries to import them with importTextFiles.php, this happens:
sam@memex:~/public_html/mediawiki/maintenance$ echo 'Test' > \#Test_page.txt
sam@memex:~/public_html/mediawiki/maintenance$ php importTextFiles.php \#Test_page.txt
Importing 1 pages...
Exception encountered, of type "InvalidArgumentException"
[25263c947f26094a28c2705c] [no req] InvalidArgumentException from line 100 of /home/sam/public_html/mediawiki/includes/deferred/LinksUpdate.php: The Title object yields no ID. Perhaps the page doesn't exist?
Backtrace:
#0 /home/sam/public_html/mediawiki/includes/content/AbstractContent.php(234): LinksUpdate->__construct(Title, ParserOutput, boolean)
#1 /home/sam/public_html/mediawiki/includes/page/WikiPage.php(2184): AbstractContent->getSecondaryDataUpdates(Title, NULL, boolean, ParserOutput)
#2 /home/sam/public_html/mediawiki/includes/import/WikiRevision.php(554): WikiPage->doEditUpdates(Revision, User, array)
#3 /home/sam/public_html/mediawiki/maintenance/importTextFiles.php(141): WikiRevision->importOldRevision()
#4 /home/sam/public_html/mediawiki/maintenance/doMaintenance.php(103): ImportTextFiles->execute()
#5 /home/sam/public_html/mediawiki/maintenance/importTextFiles.php(201): require_once(string)
#6 {main}It also doesn't give the desired result when there's a hash (or octothorpe) character within the filename, but in that situation it does at least use the preceding part of the filename for the page title.
This is with MediaWiki 1.27.0.