SEQ: do not automatically pick up files with the .seq suffix

Reported by Stéphane Dallongeville.

To test, copy any TIFF file to ```test.seq```.  With this PR, the reader used for ```test.seq``` should match the reader used for the original TIFF.  Without this PR, ```test.seq``` should be read using ```SEQReader```.  The name of the used reader can be checked using ```showinf```.