assuming your mySQL database is going to be on a UNIX/Linux type system, i suggest exporting your data into "tab delimited" text format.

UNIX/Linux systems can't read .xls that easily, the programmer would probably export it into a text file anyhow.

.csv would work, but depending on what kind of data you have, i've had troubles with data such as: "Houston, TX" or "Mr. John Doe, Jr."