Okay, I removed the text entry area in the File and Text Encryption/Decryption tab as it's seemed kind of useless. I renamed the tab to just "File Encryption/Decryption"
In their wisdom of basing file types on extensions (since the DOS ages) and never properly implementing or enforcing internal file type desination, some files use the first 4 bytes as a file type (like all PDFs start with %PDF), but many do not.
I figured out a way to "sniff" the file for clues, but came to the conclusion that it would be highly inaccurate (even notepad will try to open non-text files if it's fooled).
Therefore, I cheated.
If you check the "View Decrypted File" CheckBox, the file will open in it's native application (if there is one), similar to double-clicking on a file in windows explorer.
If the decrypted file is an ".exe" it will ignore the CheckBox and never run/open a file with an extension of ".exe".
Please test at your leisure.