Is there an indicator other than file extension that indicates the file type?

Question

I am trying to make .txt file look like a .jpg file so it can be sent across wi-fi using an Eye-Fi SD card. The card only sends .jpg files for several reasons. One reason is that the transmission path of the picture from an the SD card to the computer looks like:

Camera writes pictures to EYE-FI SD -> EYE-FI connects to local router -> local router uploads to EYE-FI servers -> EYE-FI servers upload to your computer.

[Explanation]

There could be some filter on the server end, so I found some software that allows the user to bypass the eye-fi servers so now I know I am only dealing with the SD card. It's also nice to know that no one else is looking at my files. After some experimentation, I figured out that I can put .jpg files on the card and have them transmitted once a picture is taken. I also found how that the pictures must be named in short format; a name not longer than 8 characters(excluding file extensions), this probably has to do with the fact the card is formatted in fat32 (the card can be reformatted and still works). I tried uploading a .txt file to the card and gave it a similar format, and renamed it as a .jpg file. It did transfer which indicates to me there is probably something other than a file extension which denotes how the file is formatted.

[Questions]

1) Is there someway I can spoof .txt files to make them look like .jpg files?

2) Is there some kind of program I can use (for linux) to play around with values on the card so I can figure out what triggers an upload? Any ideas one what could trigger the upload?

Answer 1

1) Yes, there are hex value in the file that indicate it is a .jpg. If you open up a .jpg file with a hex editor, you will notice that there are header lines that have a bunch of information about how the image was compress, sometimes what made the image, some firmware information etc. In the editor, you can find the string "FF D8" this indicates the beginning of image file. This is followed shortly by "FF C0". The next 6 bytes contain information about the size of the image, and (I am guessing) is used by whatever software displays the image. The end of a jpg file is denoted by the 2 byes "FF D9". Fun fact, I played around with the jpg file I was using and it seems that you can put text after the "FF D9" and still have the jpg operate. I thought this was neat. Source

None of this was needed to get the eye-fi to upload the file though. As I said in my question, the card needs the name of the file to be in short format (which means the title cannot have more than 8 characters) and needs to have an acceptable file extension, in my case I used ".jpg". I wrote a text file, and just saved it as a "text.jpg". I found that there is a minimum size required in order to transfer the file, which is strange.

My hex editor of choice for this was bless, it is good for opening files, but I have yet to figure out if it can open volumes. It doesn't seem like it can.