401890 Fix .csv import

Looking at BUG:401890 "Krita doesn't open csv archives", I noticed that the error message reported was "Krita does not support this file format", which is given for an unsupported mimetype, which is odd, because .csv was supposed to be supported. On investigation I found that Krita did not recognize the .csv format at all and defaulted to "text/plain", which of course isn't supported.

Adding the format to KisMimeDatabase::fillMimeData() lets Krita recognize it, and as far as I can tell importing does work correctly.

Test Plan

1. Save an animation as .csv. Then, modify and save it again, because the exporter tries to use the document's current filename instead of the one being saved into for creating the .frames folder, which fails when the current filename is not .csv.
2. Open the .csv file. Confirm it loads correctly.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.